SINGLE STEP SURFACE PREPARATION USING A CITRIC ACID COMPOSITION

Description

FIELD

This development relates generally to surface preparation, in particular to systems and methods for preparing surfaces using a citric acid composition for subsequent treatment such as conversion coatings.

BACKGROUND

Surfaces of rocket bodies and other structures may have a pretreatment such as a pretreatment coating, for example a chromate conversion coating. Some coatings may be used to improve the corrosion resistance of bare metal and as a primer to improve adherence of paints and adhesive. The coating may also provide resistance to abrasion and light chemical damage on soft metal. Coatings are traditionally applied to a surface by dipping, spraying, or brushing. Surfaces such as aluminum alloy surfaces require preparation prior to other treatments, such as application of a conversion coating.

Typical types of preparation include mechanical preparation or phosphoric acid or sodium hydroxide (NaOH) gel etchants that are complex to use and involve multiple steps and require a desmut containing nitric acid, hexavalent chromium, or other hazardous material. Further, typical preparations dimensionally change the substrate, which can be problematic if the substrate cannot tolerate more rework, such as etching or abrading. This is a disadvantage in use cases that operate with close tolerances and where the substrate includes areas that are not a part of the preparation area. Further, typical preparations can damage surface areas not being treated, such as areas where a chromate conversion coating is not damaged and is not in need of repair, thus requiring extra care and attention to such areas. Thus, there exists a need for improvements to surface preparation that are simple to use, produce minimal hazardous waste, allows for conversion coating application without violating dimensional tolerances of the surface, and limits damage to already treated areas of a surface.

SUMMARY

Systems and methods are described for preparation of a surface such as an aluminum alloy surface for a pretreatment such as a chemical conversion coating. Further described are example compositions for use in a simplified and reduced toxicity surface preparation. Compositions may include a citric acid based composition, which may be a gel, applied in a single step process that replaces the functions of both an etchant and a desmut in a preparation of the surface for the pretreatment.

Each embodiment disclosed herein has several aspects no single one of which is solely responsible for the disclosure's desirable attributes. Without limiting the scope of this disclosure, its more prominent features will now be briefly discussed. After considering this discussion, and particularly after reading the section entitled “Detailed Description,” one will understand how the features of the embodiments described herein provide advantages over existing approaches to preparing surfaces for coating.

In an aspect, a system for single-step preparation of an aluminum alloy surface for a chemical conversion coating includes: a composition configured to raise surface energy of the aluminum alloy surface suitably for application of the chemical conversion coating without a need to remove contaminates after removal of the composition, the composition including: an etchant including at least 50% citric acid by volume, and a thickener including at least 10% fumed silica by volume; and an applicator configured to apply the composition.

Various embodiments of the above and other aspects may be implemented. In some embodiments, the applicator includes a brush. In some embodiments, the thickener includes no more than 15% fumed silica. In some embodiments, the aluminum alloy surface is on a rocket body. In some embodiments, the composition includes a viscosity greater than 1,000 centipoise. In some embodiments, the composition includes a viscosity of no more than 250,000 centipoise.

In another aspect, a method of treating an aluminum alloy surface for a chemical conversion coating includes: degreasing the aluminum alloy surface; applying a composition to the aluminum alloy surface, the composition including at least 50% citric acid by volume and a basic thickener; agitating the composition on the aluminum alloy surface for a minimum of 3 minutes; removing the agitated composition from the aluminum alloy surface; and applying a chemical conversion coating to the aluminum alloy surface after removing the composition.

Various embodiments of the above and other aspects may be implemented. In some embodiments, the chemical conversion coating is applied within 10 minutes after removing the composition. In some embodiments, the basic thickener includes fumed silica. In some embodiments, the composition includes at least 10% fumed silica by volume. In some embodiments, the composition includes no more than 15% fumed silica by volume. In some embodiments, a normal vector of the aluminum alloy surface is angled with respect to a direction of gravity while applying and agitating the composition. In some embodiments, agitating the composition includes agitating the composition for at least 7 minutes. In some embodiments, the chemical conversion coating includes a chromate conversion coating. In some embodiments, the method further includes applying deionized water to the aluminum alloy surface after removal of the agitated composition and prior to applying the chemical conversion coating, and wherein a contact angle measurement of the deionized water on the aluminum alloy surface is less than 40 degrees. In some embodiments, the contact angle measurement is less than 30 degrees. In some embodiments, a thickness of the aluminum alloy surface is preserved to within a desired dimensional tolerance after removal of the agitated composition. In some embodiments, the aluminum alloy surface is on a rocket body.

In another aspect, a composition may be configured to raise a surface energy of an aluminum alloy surface for subsequent application of a chemical conversion coating to the surface, without a need to remove contaminates after removal of the composition, the composition including: an etchant including at least 50% citric acid by volume, and a thickener including at least 10% fumed silica by volume.

Various embodiments of the above and other aspects may be implemented In some embodiments, the thickener includes no more than 15% fumed silica by volume.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present disclosure will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only several embodiments in accordance with the disclosure and are not to be considered limiting of its scope, the disclosure will be described with additional specificity and detail through use of the accompanying drawings. In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, may be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and make part of this disclosure.

FIG. 1A is a cross-sectional schematic of an example of multi-layer corrosion protection, including an embodiment of an aluminum alloy surface prepared in a single step for a conversion coating, and that may be used to protect the aluminum alloy surface.

FIG. 1B is a cross-sectional schematic of an example surface preparation composition applied to an aluminum alloy surface using an applicator during an example surface preparation process.

FIG. 2A is a flow chart illustrating an example surface preparation process for utilizing a citric acid based composition or chemistry for a single step conversion coating process.

FIGS. 2B and 2C illustrate example contact angles of water on an example surface that may result during example water break free surface tests.

FIG. 2D illustrates front and side views of an example surface having areas of good and poor surface preparation that may result during a water break free surface test.

FIGS. 3A and 3B are data charts illustrating example resulting contact angles of compositions at different times after etching from water break free surface tests for, respectively, various concentrations of citric acid and for various etching durations.

FIGS. 4A-4C show magnified views of an example surface after an example mechanical preparation for conversion coatings involving abrading a surface until a desired surface preparation condition is reached, such as when a water break free (WBF) test is passed.

FIGS. 5A-5D show magnified views of an example surface preparation of 10% sodium hydroxide (NaOH), 10% corn starch etching over a 2 minute period and a desmut consisting of 50% citric acid and 15% fumed silica over a 3 minute period.

FIGS. 6A-6D show magnified views of an example surface preparation of 10% NaOH, 10% corn starch etching over a 2 minute period and a desmut consisting of 50% nitric acid and 10% fumed silica over a 3 minute period.

FIGS. 7A-7D show magnified views of an example surface preparation of a single step application of a citric acid composition of 50% citric acid and 15% fumed silica over a 3 minute period.

FIG. 8 is a data chart showing example resulting contact angles of ionized water on a surface as a function of minutes after surface preparation and for different compositions and methods of surface preparation.

DETAILED DESCRIPTION

The following detailed description is directed to certain specific embodiments of the development. Reference in this specification to “one embodiment,” “an embodiment,” or “in some embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. The appearances of the phrases “one embodiment,” “an embodiment,” or “in some embodiments” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others.

Various embodiments will now be described with reference to the accompanying figures, wherein like numerals refer to like elements throughout. The terminology used in the description presented herein is not intended to be interpreted in any limited or restrictive manner, simply because it is being utilized in conjunction with a detailed description of certain specific embodiments of the development. Furthermore, embodiments of the development may include several novel features, no single one of which is solely responsible for its desirable attributes, or which is essential to practicing the present disclosure.

Multi-Layer Corrosion Protection of Surfaces

Rocket bodies are exposed to conditions that may cause corrosion of materials composing the rocket body. For example, these conditions include exposure of a rocket body while on the ground to humidity, corrosive chemicals, fuels and other oxidizers. The rocket may also encounter atomic oxygen and UV light in the upper atmosphere. Thus, surfaces of rocket bodies need to be corrosion resistant.

Aluminum alloys are a common material for rocket body panels and other structures due to certain beneficial properties, such as their lightweight strength. However, aluminum alloys may be eroded by exposure to molecular or atomic oxygen in the environment.

Multi-layer corrosion protection may be used to protect the aluminum alloys of rocket bodies and other structures from corrosion. The multi-layer corrosion protection may be used for a variety of different structures, such as rockets, but also vehicles, tools, buildings, components, devices, construction materials, and other structures and items subject to corrosive environments. Further, the multi-layer corrosion protection may be used for a variety of different metals and materials, such as aluminum or aluminum alloys, but also steel, magnesium, zinc, titanium, or alloys thereof. Example types of conversion coatings or processes that may be used to passivate and/or coat a surface include, but are not limited to, chromate conversion coatings, phosphate, bluing, black oxide, anodizing, stannate, molybdate, zirconate, titanate, and plasma electrolysis.

FIG. 1A illustrates an example cross-sectional schematic of a surface 111 with a multi-layer corrosion protection that may be used to protect the surface 111 of a material. The surface 111 may be a surface of any of the structures described herein, such as a rocket body, and may be any of the materials described herein, such as an aluminum alloy. The surface 111 may be exposed to a corrosion-inducing environment when used.

A first layer on the surface 111 may include a pretreatment coating such as a conversion coating 113 as shown that protects the surface 111 from corrosion. The conversion coating 113 may act as a base for adhesion of a primer 115 and/or a topcoat 117, such as a paint, thereon. The topcoat 117 is applied over the primer 115 and acts as an additional layer of protection to the surface 111.

The conversion coating 113 may be a pretreatment that converts properties of the surface 111 by a chemical conversion process. The conversion coating 113 may penetrate and/or passivate the surface 111. The conversion coating 113 may produce a metal oxide layer on the surface 111 that protects the surface 111 from corrosion (such as rust, for example) and other sources of damage. The conversion coating 113 may be a chromate conversion coating, a chromate-phosphate-based conversion coating, or other type of coating such as those described herein.

Preparation of Surfaces for Conversion Coating

Certain materials for the surface 111, such as aluminum alloys, may require surface preparation prior to application of the conversion coating 113. This preparation may remove any native oxide and residual organic and nonorganic material on the surface 111. Traditional chemistries for this process may react with the surface 111 to dissolve the material matrix, alloy elements, and/or modify the surface finish. Such an aggressive approach is undesirable when repairing a conversion coating on surfaces that are, for example, rocket body components that are already at final tolerances and/or that must maintain a specific surface finish. Further, such an approach includes a risk of the chemistries used for repairing surfaces dripping onto areas not being repaired. Due to the nature of the traditional chemistries, such dripping can damage the surface and/or equipment involved in repair. The citric acid gel methods according to the present disclosure will not result in accidental damage if dripped or accidentally released onto surrounding areas not being repaired.

Other repair methodologies for final surfaces rely on mechanical methods, such as a light abrasion, but mechanical methods also have drawbacks. Highly textured surfaces, areas of frequent repair, and restrictive geometry inhibit surface preparation by mechanical methods. For example, flight hardware with specific topographical finishes (such as a gore panel with a shot-peened surface) and rigid dimensional requirements require a different method to prepare damaged surfaces for application of conversion coatings. The mechanical abrasion technique is not ideal due to the effort required to clean out textured areas and the technique also removes or damages the texture. Some areas may require multiple rework attempts which quickly result in the removal of the maximum allowed material for tolerance requirements. Further, mechanical preparation cannot penetrate deep into damaged areas, such as surface scratches. Thus, mechanical preparation can be insufficient for scratch repair. Additionally, accidental abrasion of surfaces during mechanical preparation can cause unwanted surface damage in areas not intended for repair.

In contrast, the surface preparation systems and methods according to the present disclosure provide various advantages. For instance, the single step conversion approach described herein may prepare the surface for subsequent application of a chemical conversion coating while preserving the surface finish and dimensions of final surfaces, remove debris and oxidation on the surface, infiltrate complex topography, minimize material removal, reduce environmental health and safety risk and exposure, that is simple to apply, and/or that meets or exceeds existing surface preparation methods in qualification testing. A thickness of an aluminum alloy surface may be preserved to within a desired dimensional tolerance after removal of the agitated composition. Such dimensional tolerance may be no more than +/−0.001″ (inches), no more than +/−0.0005″, or no more than +/−0.0001″.

Systems and methods for surface preparation according to the present disclosure include preparation of a surface, such as an aluminum alloy surface, for application of a corrosion resistant coating, such as a chromate conversion coating. The systems and methods for surface preparation may include a citric acid based composition configured to perform the functions of both an etchant and desmut in preparation of the surface, as further described herein.

Example Citric Acid Based Compositions for Surface Preparation

Example compositions according to the present disclosure for preparing the surface 111 for the conversion coating 113 include compositions 119 that may act as both an etchant and desmut. FIG. 1B illustrates an example application of the composition 119, embodied as a gelled citric acid composition, to the surface 111 by an applicator 121. The surface 111 prepared with the composition 119 may be a surface of an aluminum alloy. The conversion coating 113 may be a chromate conversion coating. However, other examples of surfaces and/or conversion coatings may also be applicable to the systems, methods, and applications of the compositions disclosed herein.

The composition 119 includes some concentration of citric acid in a solution in combination with one or more other materials. In some examples, the composition 119 may include a basic thickener, such as fumed silica. A basic thickener may include a material that is non-acidic or having a pH of greater than 7. The basic thickener may be of a high enough concentration to generate a gel-like consistency of the composition 119 at room temperature. In some non-limiting examples, the composition 119 may include a gelled composition of approximately 50% citric acid combined with 10-15% fumed silica. The composition may include 10% or about 10% fumed silica, 15% or about 15% fumed silica, at least 10% fumed silica, at least 11% fumed silica, at least 12% fumed silica, at least 13% fumed silica, at least 14% fumed silica, or at least 15% fumed silica. As further example, the composition may include from 4-21%, from 6-19%, from 8-17%, from 10-15%, or from 12-14%, fumed silica.

The composition 119 may have a Newtonian or non-Newtonian viscosity. The citric acid-based composition may have a viscosity between approximately 1,000 and 250,000 centipoise (cps), for example 5,000 cps or approximately 5,000 cps., or from 3,000 cps to 7,000 cps. The viscosity may provide a consistency to the composition 119 that allows for application of the composition 119 to a surface using the applicator 121, such as a brush as shown, or a paint stick, roller, foam, sponge, or other manual or automatic applicator. A spray applicator may additionally or alternatively be used. The viscosity may allow the surface 111 to remain covered or in contact with the composition for the desired period of application in a variety of orientations of the surface 111. For example, the consistency should be such that when the surface 111 is non-perpendicular to the direction of gravity, such as vertical to a horizontal ground plane, the composition remains on the surface 111.

An example preparation method of the composition 119 may include diluting a citric acid powder to 50% and thickening the resulting solution with a basic thickener, such as 15% fumed silica. A solvent for diluting the citric acid powder may include deionized water or another solvent. The process of application of the composition 119 may include but is not limited to: less than 10 minutes of exposure of the composition 119 to the surface, regular and/or constant agitation of the composition 119 on the surface, and/or wiping of the composition 119 off of the surface using deionized water, as further described herein.

While many methods may be able to obtain a clean and active surface as discussed above, citric acid based compositions have certain benefits over more hazardous and/or traditional preparation methods such as mechanical abrasion and/or multi-step sodium hydroxide (NaOH) etching. For example, advantages of the composition 119 over mechanical or other acidic etchants, such as NaOH etchant, include, but are not necessarily limited to, providing an environmentally-friendly cleaner for anodized surfaces that may remove native oxide, loose material, and generate an active surface for conversion coatings while not disturbing intermetallics on the surface of metal alloys and not introducing corrosion initiation sites. Additionally, the composition 119 may be used in areas that cannot undergo any more dimensional changes or where the surface topography must remain intact, both of which may be problematic in the case of mechanical abrasion techniques for preparation of a surface for conversion coating. Another example advantage of the composition 119 is that it need only do the work of removing the native oxide and generating an active surface for the conversion coating as imperfections and mills on the surface 111 would typically have already been previously removed using a different etchant or other preparatory process. Additionally, when the preparation of the surface 111 is complete, the composition 119 may be removed with a deionized water wipe and need not necessarily be treated as hazardous waste, reducing waste and easing use.

Example Preparation Processes Using Citric Acid Based Compositions

FIG. 2A illustrates an example process 200 for preparing a surface 111 utilizing a citric acid based composition. The process 200 may be performed by an operator, such as a user, multiple users, a machine, or a combination thereof. One or more steps of the process 200 may be excluded in some instances. Additionally or alternatively, the process 200 may include more, fewer, or different steps than described herein. One or more steps of the process 200 may additionally be repeated, such as application, agitation, and removal of a composition, as further described.

The process 200 may be used with the surface 111, such as a metallic surface, for example as described with reference to FIGS. 1A and 1B. The surface 111 may be part of a rocket body that cannot undergo mechanical abrasion techniques for preparation of a surface due to tight dimensional tolerances that require minimal material removal or displacement. Additionally, some surfaces may be ill-suited to mechanical or other preparation techniques due to difficult surface topography, such as a shot peened surface of a rocket body panel. The process 200 may be performed in whole or in part while the surface 111 is in any number of orientations, including a vertical orientation or other angled orientation with respect to a gravity vector.

The surface 111 may or may not have previously gone through one or more previous surface preparations prior to performing the process 200. For example, in some cases, the surface 111 may have gone through a preliminary etching or preparation to remove imperfections and/or mills on the surface. The preliminary etching may, for example, have been performed during manufacture and/or assembly of a rocket body that includes the surface 111. In other examples, the surface preparation may have previously included one or more steps of the process 200. The surface preparation may have been previously completed and a conversion coating applied, and the process 200 may be used to treat the surface for repair of the conversion coating and/or other treatments of the surface.

The process 200 includes block 201, where the surface 111 is degreased. An operator may degrease the surface 111. The surface 111 may be degreased to be prepared for conversion coating. Degreasing the surface 111 may involve ridding the surface 111 of oils and/or drying the surface 111. In some examples, the operator may apply isopropyl alcohol or an isopropyl alcohol-based solvent to the surface 111. The application may involve wiping the surface 111 with a presoaked wipe and/or covering the surface 111 in the solvent and wiping it away. The surface 111 may additionally be dried using a separate cloth and/or other method prior to application of the chemistry in block 203.

The process 200 then moves to block 203, where the composition 119 is applied to the surface 111. The composition 119 may be applied by any of the methods described herein, for example an operator may apply the composition 119. The composition 119 may be applied at a desired location. The desired location may include specific areas of the surface 111 needing repair of chemical conversion coating and/or other treatments. The location may include a larger area of the surface 111, such as with a preparation for an initial conversion coating. The application may include brushing the surface 111 with a polyethylene (PE) brush containing the composition. A normal vector of the surface may be angled with respect to a direction of gravity while applying the composition 119.

The composition 119 may include any of the various compositions described herein, for example a gelled citric acid based composition that includes a 50% citric acid based solution with 10% to 15% fumed silica, or any of the other compositions as described herein. In some embodiments, the operator may use a premade composition. In some embodiments, the operator may prepare and make the composition 119 as part of the process 200 before the application at block 203, and/or before or after degreasing at block 201.

The process 200 then moves to block 205, where the applied composition 119 is agitated on the surface 111. The operator may agitate or disturb the applied composition 119. The operator may agitate the composition 119 for at least a portion of the full period of time during which the composition 119 sits on the surface 111 before removal (e.g., at block 207), sometimes referred to as the “period of application”. In some embodiments, the operator may agitate the composition 119 on the surface 111 for the full period of application. In some embodiments, the operator may periodically agitate the composition 119 throughout at least a portion of the full period of application. In other examples, the operator may agitate the composition 119 and/or disturb the composition 119 on the surface 111 randomly, once, or non-periodically during the period of application. The method of agitation may include disturbing the composition 119 on the surface 111 at the applied area. The method of distribution or disturbing of the composition 119 may include but is not limited to utilizing the same or different tool as used in the application at block 203 to move the composition around the applied area. The agitation may include moving the composition 119 in a pattern on the surface 111, such as swirling the composition 119 on the surface 111 at the applied area. The agitation may include swirling, mixing, brushing, whipping, flattening, other movements of the composition 119, or combinations thereof. A normal vector of the surface may be angled with respect to a direction of gravity while agitating the composition 119.

The process 200 then moves to block 207, where the composition 119 is removed from the surface 111. The operator may remove the composition 119. The method of removal may include wiping, scraping, and/or rinsing the composition 119 from the surface 111. In one example, the operator may scrape the composition 119 off the surface 111 and rinse the surface 111 with, for example, a wipe and/or application of deionized water. The composition 119 may be scraped from the surface 111 using a plastic scraping tool.

The length of time from block 203 (application) to block 207 (removal) may sometimes be referred to herein as a period of application and may include, but is not limited to a period of at least 3, at least 5, at least 7, or at least 9 minutes. The choice of the period of application may be based on and/or associated with a desired surface activation level as measured by a water break free surface test. The choice of the period may additionally be based on other factors, such as workflow considerations.

The end result of the block 207 may include a clean and activated surface 111. An activated surface 111 includes the substantial removal of the native oxide on the metallic surface. An activated surface 111 may be defined in the context of a water break free surface test, also known as a water break test. A water break free surface test may include a test involving measuring a contact angle of deionized water on the prepared surface 111. The test may include utilizing a surface analyzer with a few drops of deionized water. The lower the contact angle of deionized water on the prepared surface 111, the more suitable or activated the surface 111 is for a conversion coating. A measured angle greater than about 40 degrees may be unsuitable for conversion coating. As the surface preparation involves removing the native oxide of the surface, which may return with time, the characterization and/or contact angle may change with time, increasing as more time elapses after the completion of the process 200. Examples of resulting surfaces and contact angle measurements for different preparation techniques respectively are discussed with reference to FIGS. 4A-8, with contact angle measurements as a function of time illustrated in FIG. 8.

The surface 111 may therefore be prepared in a “single step” using the single composition 119. The composition 119 may be applied only one time and then removed after agitation without requiring any additional application of the composition 119 or other materials for preparation. After removal, the surface 111 may be suitable for application of the conversion coating 113, such as illustrated in FIG. 1A, without the need for further application of chemicals, cleaning agents, etchants, or the like. Because the composition 119 can sufficiently remove a native oxide from the surface without the production of reaction products that remain on the surface (or “smut”) that need to be removed with another chemical composition, such as a desmut, the process effectively functions as both an etchant and a desmut, and there is no need to additionally apply an etchant or a desmut. Thus, the use of the composition 119 facilitates preparation of the surface 111 for a conversion coating in a single step. The single step may involve blocks 203, 205 and 207. The single step according to the present disclosure is in contrast to traditional methods that involve application of a first material, removal of the first material, and then application of a second material, in order to prepare the surface for the pretreatment.

The process 200 then moves to block 209, where the pretreatment such as the conversion coating 113 is applied to the surface 111. The operator may apply the pretreatment. The pretreatment may be applied to the clean and activated surface 111. The pretreatment may include, but is not limited to, a chemical conversion coating such as a chromate conversion coating described with reference to FIGS. 1A and 1B. Further treatments may additionally be applied, such as a primer and/or paint. The pretreatment and/or treatment may be applied using, for example, a cloth or other method.

In some examples, prior to block 209, a water break free surface test such as described above may be performed to identify readiness of the surface 111 for the treatment. The operator may perform the test. If the surface 111 does not pass the water break free surface test, the operator may perform one or more of the blocks 201, 203, 205, 207 again until a water break free surface test is passed and/or as desired. Further description of an example water break free surface test is discussed herein with reference to FIGS. 2B-2D.

In some examples, after block 209, the operator may perform a corrosion test, such as a salt fog, after a period of time (for example, 24 hours after application of the pretreatment and/or treatment). The operator may additionally use the same or other methods for touch up of the surface 111.

Example Contact Angle Tests

The effectiveness of a surface preparation composition, and resulting suitability of the surface for a conversion coating, can be assessed through a water break free surface test. A water break free surface test is a measure of whether and to what extent a surface is contaminated and/or is a measure of the surface energy of the surface. A water break free surface test may involve measurement of the degree of wetting, such as by a measure of the equilibrium contact angle of a sessile droplet of a fluid, typically distilled water, on the surface. The angle between the liquid-solid interface and the liquid-vapor interface of the sessile droplet defines the Young's or equilibrium contact angle. When a drop of distilled water is placed on a surface and reaches equilibrium (e.g., such that the drop is sessile or no longer spreading and has not substantially evaporated), the equilibrium contact angle can be measured. This measurement of contact angle can be part of the water break free surface test.

FIGS. 2B-2D illustrate representative views of results of an example water break free surface test. FIGS. 2B and 2C illustrate side views respectively of a drop 213, 217 of distilled water in equilibrium on horizontal surfaces 211, 215. The equilibrium contact angle θ1 of drop 213 on surface 211 is greater than the equilibrium contact angle θ2 of drop 217 of the surface 215. Where the contact angle is greater, as illustrated in FIG. 2B, the surface energy is lower and thus the surface 211 is more hydrophobic than the surface 215. Where the contact angle is lower, as illustrated in FIG. 2C, the surface energy is higher and the surface 215 is more hydrophilic than the surface 215. For purposes of application of a pretreatment such as the conversion coating, a well-prepared surface should have a higher surface energy and/or be more hydrophilic. Thus, the lower the contact angle, the more suitable the surface is for a conversion coating, and the more effective is the surface preparation process. As a quantitative measure, a measured contact angle should be less than 40 degrees, or less than approximately 40 degrees, to be suitable for conversion coating. In using the single step, citric-acid based composition embodiments according to the present disclosure, such as the composition 119, the resulting contact angle of deionized distilled water applied to such surface after preparation may be no more than 50 degrees, no more than 45 degrees, no more than 40 degrees, no more than 35 degrees, or no more than 30 degrees. The contact angle may be measured by a surface analyzer or other surface inspection device.

Another type of water break free surface test for measuring the suitability of the surface for application of a conversion coating can involve a more qualitative assessment of surface free energy by observing the formation of droplets via application of large volumes of water as opposed to just one drop. The droplets may form after pouring water on the surface, such as illustrated in FIG. 2D, as opposed to resulting in a more complete or continuously spread out wet layer over the surface when exposed to the water. Distilled water may be used. The qualitative test of FIG. 2D may thus include pouring more than just a drop of water onto the surface 221, while the quantitative tests of FIGS. 2B and 2C may only involve applying a single drop or multiple single drops.

FIG. 2D illustrates a top view 225A and a side view 225B of an example surface 221 with water 220 poured thereon. The surface 221 has two different surface energies in two different regions 222, 223. A large enough volume of the water 220 is applied to cause the water to either form multiple droplets or to spread out in a layer, depending on the surface energy of the surface 221. The first region 222 of the surface 221 has a relatively lower surface energy as compared to the second region 223 of the surface 221 which has a relatively higher surface energy.

As illustrated in FIG. 2D, in this particular qualitative water break free surface test, the water 220 that was poured onto the surface 221 forms, via visual inspection, droplets 219 on the surface 221 in the first region 222, and spreads out to form a thin film 224 such as a sheet or layer in the second region 223. Thus, the surface 221 in the first region 222 has a lower surface energy than in the second region 223. The first region 222 may contain contaminants and/or not be sufficiently prepared for application of a conversion coating.

If the water 220 at least partially wets the surface 221 with the thin film 224 as shown in the second region 223 of FIG. 2D, the surface 221 has a higher surface energy and is less likely to contain contaminants and/or thus may be suitable for application of a conversion coating. Thus, if the water “breaks” into droplets 219, the surface may not be suitable for conversion coating. It is of note, however, that a water break free surface test is only suitable to detect hydrophobic residues or contaminants and thus may not identify all types of contaminants a user may desire to remove from a surface prior to application of a conversion coating.

FIGS. 3A and 3B are data charts illustrating example measurements from a water break free surface test for citric acid based compositions at different dwell times and/or times after etching. The measurements may result after using any of the various compositions as described herein, such as the composition 119, and in any of the processes described herein for preparing the surface 111 for pretreatment. The various methods for measuring the contact angle as described herein may be used to determine the contact angle.

The amount of time after surface preparation for which a surface is allowed to sit may be referred to as “time after etching.” The contact angle as measured by, for example, a water break free surface test, may change (increase or decrease) with increased time after etching. This is a result of gradual oxidation of the prepared surface after the native oxide is removed during surface preparation and happens as a result of exposure to ambient air. As oxidation occurs, the surface energy of the surface decreases and/or the surface is no longer considered “active”. For example, FIG. 3A illustrates contact angle measurements just after etching (such as after block 207 in the process 200 of FIG. 2A), as well as 5 minutes, 10 minutes, and 15 minutes after etching, for citric acid compositions having concentrations of 20%, 35%, and 50%. As shown, for all three compositions, the surface is active (as shown by having a contact angle less than 25 degrees in a water break free surface test) for at least up to 15 minutes after etching and thus a conversion coating may be applied effectively within that time period. As shown, the measured contact angle just after etching may be less than approximately 15 degrees for all three compositions. The measured contact angle 5 minutes after etching may be less than approximately 20 degrees for all three compositions. The measured contact angle 10 minutes after etching may be less than approximately 22 degrees for all three compositions. The measured contact angle may be less than approximately 25 degrees for all three compositions at 15 minutes after etching.

In particular, as shown in FIG. 3A, for the composition 119 having a citric acid concentration of 20%, the contact angle may be less than 15 degrees just after etching, less than 20 degrees at 5 minutes after etching, less than 25 degrees at 10 minutes after etching, and less than 25 degrees at 15 minutes after etching. For the composition 119 having a citric acid concentration of 35%, the contact angle may be less than 15 degrees just after etching, less than 15 degrees at 5 minutes after etching, less than 20 degrees at 10 minutes after etching, and less than 20 degrees at 15 minutes after etching. For the composition 119 having a citric acid concentration of 50%, the contact angle may be less than 15 degrees just after etching, less than 20 degrees at 5 minutes after etching, less than 20 degrees at 10 minutes after etching, and no more than 20 degrees at 15 minutes after etching.

The amount of time during which the surface is actively in contact, and being prepared, with the composition 119 may be referred to as “etch time” (also referred to as period of application, or the amount of etching time). The etch time can include the duration of agitation, brushing, mixing, etc., with use of one or more applicators as described herein, such as the applicator 121. FIG. 3B illustrates an example of resulting contact angle measurements for a given citric acid composition for etch times of 1 minute, 3 minutes, 6 minutes and 9 minutes, and for measurement times after etching including just after etching (such as after block 207, denoted in FIG. 3B as 0 minutes), and 5 minutes, 10 minutes, and 15 minutes after etching. The given citric acid solution in the example shown in FIG. 3B includes 50 percent citric acid solution thickened with 15 percent by weight fumed silica. As shown, for an etch time of 1 minute, the contact angle may be less than 20 degrees just after etching, less than 25 degrees at 5 minutes after etching, less than 30 degrees at 10 minutes after etching, and less than 25 degrees at 15 minutes after etching. For an etch time of 3 minutes, the contact angle may be less than 15 degrees just after etching, no more than 15 degrees at 5 minutes after etching, less than 20 degrees at 10 minutes after etching, and less than 20 degrees at 15 minutes after etching. For an etch time of 6 minutes, the contact angle may be less than 15 degrees just after etching, less than 20 degrees at 5 minutes after etching, less than 20 degrees at 10 minutes after etching, and no more than 15 degrees at 15 minutes after etching. For an etch time of 9 minutes, the contact angle may be less than 10 degrees just after etching, less than 15 degrees at 5 minutes after etching, less than 20 degrees at 10 minutes after etching, and less than 15 degrees at 15 minutes after etching.

Therefore, for this particular embodiment, an etch time of 3 minutes may be within two standard deviations of an etch time of 9 minutes. Thus, an etch time of as little as 3 minutes may be sufficiently effective in surface preparation for a conversion coating. However, an etch time greater than 3 minutes may be used depending on the desired balance of time efficiency for surface preparation versus the desired level of surface preparation (such as necessitated by the amount of native oxide to remove or level of hydrophilicity desired).

Example Comparisons of Single Step Citric Acid Compositions versus Traditional Multi-Step Processes

This section discusses comparisons of surface preparation with citric acid compositions according to the present disclosure, such as the composition 119 and using the process 200, as compared to traditional methods such as mechanical preparation, phosphoric acid gel etchants, and/or NaOH gel etchants.

Traditional mechanical preparation utilizes abrasives to sand a surface 111 for removal of native oxide and other material on the surface 111. The method may include removal of oils from the surface 111 by, for example, wiping the surface with isopropanol, mechanically abrading the surface 111 using an abrasive, such as by a handheld abrasive, in at least two directions on the surface 111, and wiping the surface 111 with isopropanol to remove the abraded material. This method is operator-dependent and creates a dimensional change of the surface 111, removing the current surface topography and replacing it with a divergent surface. This preparation may have limited intermetallic disruption and be relatively simple for an operator. It is advantageous to limit intermetallic disruption in the surface material because intermetallic presence on the surface can make an aluminum alloy surface susceptible to undesirable localized corrosion. However, mechanical abrasion cannot be used on specific topography or certain part designs and may result in uneven abrasion patterns and inconsistent material or oxide removal.

FIGS. 4A-4C show magnified views of an example surface after an example mechanical preparation for conversion coatings, in contrast to the preparation according to the present disclosure. The mechanical preparation of FIGS. 4A-4C involves abrading a surface until a desired surface preparation condition is reached, such as when a water break free surface test is passed, as described herein.

As shown in the secondary electron (SE) image of FIG. 4A, the resulting surface from an example mechanical preparation method has generated a clean surface with no dealloying of intermetallics, intermetallic removal, or trench etching. Limiting intermetallic disruption may reduce localized corrosion by, for example, reducing sites of potential corrosion on the surface. For example, FIG. 4B shows a backscattered electron (BSE) image of the mechanically abraded surface of FIG. 4A, showing surface copper (Cu) particles 405 present after mechanical abrasion, with minimal or no pitting or cavities in the surface.

However, as illustrated in FIGS. 4A and 4B, grooves and patterns 401 are present as a result of the mechanical abrasion process. These grooves or patterns occur as a result of the mechanical abrasion method, which may unevenly remove surface material using a mechanical abrasive to remove the surface oxide. As noted above, this type of surface texture can be problematic due to the effort required to clean out the textured areas and/or the damage to the surface.

FIG. 4C shows oxygen density of the surface shown in FIGS. 4A and 4B. The varying levels of oxygen density are shown by the relatively lighter and darker areas. Increased oxygen density is associated with the brighter areas of the image. The oxygen density appears in the brighter non-grooved areas, indicating the presence of native oxide in the non-grooved areas, while the less bright grooved areas are relatively clean. Thus, while mechanical abrasion does generate a clean surface, oxide removal is not necessarily uniform.

As another example of a traditional preparation method, an NaOH etching process is a multi-step process. A first step of NaOH etching involves using a NaOH based gel etchant to etch the surface 111. A second step of NaOH etching involves using an acid desmut to remove contaminants resulting from the etching to complete surface preparation. In some examples, a NaOH etchant may include a composition of 10% or 20% NaOH and a basic thickener, such as corn starch (for example, 7.5% corn starch). Utilizing an NaOH etching process for preparation of a surface may include removal of oils from the surface 111 by, for example, wiping the surface with isopropanol, applying an etchant (such as at a 10% concentration of NaOH) via a manual application, such as with a polyethylene (PE) brush, removal of the etchant and repeating application of the etchant multiple times over a period of application (for example, every 20 seconds over a period of 2 minutes), applying a stronger concentration etchant (such as at a 20% concentration of NaOH), removal of the stronger concentration etchant, and rinsing of the surface 111 with deionized water soaked wipes.

This preparation may provide a clean surface. However, the process may introduce corrosion initiation sites that reduce the efficiency of the applied conversion coating and be environmentally hazardous. Further, NaOH etching may require a desmut containing constituent chemistry containing hazardous materials such as hexavalent chromium and/or high concentrations of nitric acid.

FIGS. 5A-5D show magnified views of an example traditional surface preparation of 10% NaOH and 10% corn starch, etching over a 2 minute period, and a desmut consisting of 50% citric acid and 15% fumed silica applied over a 3 minute period.

As shown in FIG. 5A, the preparation method may generate a surface with etch pits 501 associated with Cu exposed from the surface etching and with trench etching 503 around intermetallics. FIG. 5B shows evenly distributed surface oxygen interspersed with high density areas of surface oxygen associated with the pits 501 shown in FIG. 5A. The high density areas indicate the presence of native oxide at the etch pits and exposed surface Cu that may reduce the efficiency of the applied conversion coating by providing sites for corrosion.

FIGS. 5C and 5D show respectively secondary electron (SE) imaging and back-scattered electron microscope (BSE) imaging of an example surface after preparation with NaOH and a citric acid desmut. FIG. 5D (SE image) shows light alkaline etch pits 507. FIG. 5C (BSE image) indicates the process attacked intermetallics in two ways: (1) complete removal leaving deep pits, and (2) dealloying leaving behind Cu particles 509 (shown in FIGS. 5A and 5D). Oxide presence is heavily associated with the Cu particles 509 and the open pits 507. Additionally, etching has occurred around Cu leaving trenches 505, 511 surrounding the Cu particles 509, which may initiate crevice corrosion.

FIGS. 6A-6D show magnified views of a resulting surface after a traditional preparation process using NaOH and corn starch. The surface preparation used 10% NaOH and 10% corn starch, etching over a 2 minute period, and a desmut consisting of 50% nitric acid and 10% fumed silica applied over a 3 minute period. FIG. 6A shows an SE image of the surface after such preparation. As shown in FIG. 6A, the preparation may generate a surface with etch pits 603 with Cu exposed from the surface etching, and with trench etching around intermetallics. FIG. 6B shows an example BSE image of the surface after preparation. As shown in FIG. 6B, the nitric acid desmut appears to have removed more surface Cu 607 after the NaOH etch, with trench etching 609 shown around intermetallics. FIG. 6C illustrates the surface with removed surface Cu showing pits 611 left behind with trench etching. Nitric acid appears to have removed trench etched particles more completely compared to the citric acid desmut shown in FIGS. 5C-5D. FIG. 6D illustrates surface oxygen distribution indicating evenly distributed oxygen with high density areas associated with particles and pits 611. The high density areas appear less dense compared to the citric acid desmut used in FIGS. 5C-5D. But the method has resulted in Cu pits and trench etching around intermetallics as described, which may produce corrosion.

FIGS. 7A-7D show magnified views of an example surface preparation of a single step application according to an embodiment of the present disclosure. In the illustrated example, a citric acid composition of 50% citric acid and 15% fumed silica was applied with an etch time of 3 minutes. As shown in FIG. 7A, the example single step preparation method may generate a surface with empty etch pits 701 or with remnant Cu exposed from the alloyed intermetallics. FIG. 7B shows evenly distributed surface oxygen with high density areas of surface oxygen associated with the particles and pits shown in FIG. 7A, implying the presence of native oxide at the etch pits 701 and exposed surface Cu.

FIGS. 7C and 7D show additional back imaging of an example surface after single step citric acid preparation. FIG. 7C shows the minimal etch pits 705 and trench etching 703. FIG. 7D shows that the surface Cu has been removed and a reduction of oxygen compared to the above traditional methods, indicating a relatively oxide free surface with minimal trench etching 707. The process is thus compatible with application of a conversion coating and has the potential to reduce the possibility of corrosion.

Advantageously, as shown in FIGS. 7A-7D, citric acid preparation methods better preserve the surface being prepared in comparison to other more traditional methods, such as shown in comparison to FIGS. 4A-6D. For example, citric acid preparation methods tend to generate fewer pits, in some cases up to 30 percent fewer pits than mechanical methods for Aluminum Alloy 2219 in the context of repairs. This is important for scratched surface repair and/or repairs on surfaces with low tolerance for damage, such as described herein. Citric acid compositions may additionally be able to prepare a scratched surface, deep in the scratch, allowing for better conversion coating application over other methods, such as mechanical preparation, which may not be able to properly treat and/or penetrate a deep scratch without further damaging the treated surface. For example, citric acid preparation methods may be able to penetrate a scratch while limiting damage and/or pitting in the area surrounding the scratch being repaired.

FIG. 8 is a data chart showing example contact angles of deionized water on a surface as a function of time after various surface preparation methods. The angles may be measured using the water break free surface methods described herein. The results of surface preparation from different methods may be characterized using the contact angle measurement with deionized water. A lower contact angle implies a cleaner and more active surface and a higher surface energy, as mentioned.

The different compositions and methods of surface preparation used to produce the data in FIG. 8 include the following: 10% NaOH etching followed by a nitric acid desmut according to a traditional method, a 10% NaOH etching followed by a citric acid desmut according to a traditional method, a 50% citric acid based composition single step preparation according to the present disclosure (such as the composition 119), and a mechanical abrasion preparation according to a traditional method. As shown, the citric acid based composition with single step preparation results in a surface energy of the prepared surface for the majority of the time scales that is at least comparable to other methods of surface preparation based on contact angle measurement. This implies that citric acid based compositions described herein performs about as well or better as other more hazardous and/or traditional preparation methods, such as mechanical abrasion and/or NaOH etching, for preparing a surface for application of a conversion coating. For example, all four displayed methods demonstrate a contact angle of less than approximately 40 degrees at up to approximately 30 minutes past surface preparation, with lower contact angles at shorter times right after surface preparation.

In particular, in the example shown in FIG. 8, for 10% NaOH etching followed by a nitric acid desmut according to a traditional method, the contact angle may be less than 55 degrees at pre-surface preparation, less than 15 degrees at 0.1 minutes after surface preparation, less than 15 degrees at 2 minutes after surface preparation, less than 20 degrees at 4 minutes after surface preparation, less than 25 degrees at 6 minutes after surface preparation, less than 25 degrees at 8 minutes after surface preparation, less than 30 degrees at 16 minutes after surface preparation, less than 35 degrees at 24 minutes after surface preparation, less than 40 degrees at 32 minutes after surface preparation, less than 40 degrees at 40 minutes after surface preparation, less than 45 degrees at 48 minutes after surface preparation, less than 45 degrees at 56 minutes after surface preparation.

Further, in the example shown in FIG. 8, for 10% NaOH etching followed by a citric acid desmut according to a traditional method, the contact angle may be less than 55 degrees at pre-surface preparation, less than 12 degrees at 0.1 minutes after surface preparation, less than 20 degrees at 2 minutes after surface preparation, less than 25 degrees at 4 minutes after surface preparation, less than 30 degrees at 6 minutes after surface preparation, less than 30 degrees at 8 minutes after surface preparation, less than 35 degrees at 16 minutes after surface preparation, less than 35 degrees at 24 minutes after surface preparation, less than 40 degrees at 32 minutes after surface preparation, less than 45 degrees at 40 minutes after surface preparation, less than 45 degrees at 48 minutes after surface preparation, less than 50 degrees at 56 minutes after surface preparation.

Further, in the example shown in FIG. 8, for 50% citric acid based composition single step preparation according to the present disclosure (such as the composition 119), the contact angle may be less than 50 degrees at pre-surface preparation, less than 15 degrees at 0.1 minutes after surface preparation, less than 25 degrees at 2 minutes after surface preparation, less than 30 degrees at 4 minutes after surface preparation, less than 30 degrees at 6 minutes after surface preparation, less than 35 degrees at 8 minutes after surface preparation, less than 40 degrees at 16 minutes after surface preparation, less than 40 degrees at 24 minutes after surface preparation, less than 45 degrees at 32 minutes after surface preparation, less than 50 degrees at 40 minutes after surface preparation, less than 50 degrees at 48 minutes after surface preparation, less than 60 degrees at 56 minutes after surface preparation.

Further, in the example shown in FIG. 8, for mechanical abrasion preparation according to a traditional method, the contact angle may be less than 70 degrees at pre-surface preparation, less than 25 degrees at 0.1 minutes after surface preparation, less than 25 degrees at 2 minutes after surface preparation, less than 25 degrees at 4 minutes after surface preparation, less than 30 degrees at 6 minutes after surface preparation, less than 30 degrees at 8 minutes after surface preparation, less than 30 degrees at 16 minutes after surface preparation, less than 35 degrees at 24 minutes after surface preparation, less than 35 degrees at 32 minutes after surface preparation, less than 40 degrees at 40 minutes after surface preparation, less than 45 degrees at 48 minutes after surface preparation, less than 45 degrees at 56 minutes after surface preparation.

Terminology

The flow chart sequences are illustrative only. A person of skill in the art will understand that the steps, decisions, and processes embodied in the flowcharts described herein may be performed in an order other than that described herein. Thus, the particular flowcharts and descriptions are not intended to limit the associated processes to being performed in the specific order described.

While the above detailed description has shown, described, and pointed out novel features of the present disclosure as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the device or process illustrated may be made by those skilled in the art without departing from the spirit of the present disclosure. As will be recognized, the present disclosure may be embodied within a form that does not provide all of the features and benefits set forth herein, as some features may be used or practiced separately from others. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

The term “comprising” as used herein is synonymous with “including,” “containing,” or “characterized by,” and is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. With respect to the use of any plural and/or singular terms herein, those having skill in the art may translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.

It will be understood by those within the art that, in general, terms used herein are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to embodiments containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”), and the same holds true for the use of definite articles used to introduce claim recitations.

In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.”

Unless indicated to the contrary, the numerical parameters set forth in the specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should be construed in light of the number of significant digits and ordinary rounding approaches. For example, terms such as about, approximately, substantially, and the like may represent a percentage relative deviation, in various embodiments, of +1%, +5%, +10%, or +20%.

The above description discloses several methods and materials of the present disclosure. The present disclosure is susceptible to modifications in the methods and materials, as well as alterations in the fabrication methods and equipment. Such modifications will become apparent to those skilled in the art from a consideration of this disclosure. Consequently, it is not intended that the present disclosure be limited to the specific embodiments disclosed herein, but that it covers all modifications and alternatives coming within the true scope and spirit of the present disclosure.