METHOD FOR FORMING COATING LAYER ON SUBSTRATE AND COATING LAYER FORMED THEREBY

Description

FIELD

The present disclosure relates to a method for forming a coating layer on a substrate and a coating layer formed thereby.

BACKGROUND

Magnesium alloys have been widely used across various industries because these alloys are lightweight, and have high strength and excellent mechanical properties. However, the magnesium alloys are susceptible to corrosion, wear, and other environmental damage. In order to enhance performance and durability of the magnesium alloys, various surface treatments, such as an anodizing treatment and a micro-arc oxidation (MAO) treatment, have been developed.

The anodizing treatment is an electrolytic process used to increase a thickness of an oxide film or a coating layer on a surface of a metal part. Such oxide film or coating layer is porous, so that sealing is required to fill the pores thereof. However, a biphasic effect of the magnesium alloys often results in uneven and thick oxide films or coating layers, which can adversely affect the mechanical properties, electrical conductivity, and electromagnetic shielding of the magnesium alloys.

The MAO treatment is a surface treatment similar to the anodizing treatment, but with a simpler process and less environmental impact. The MAO treatment uses an alkaline electrolyte, which is different from a strong acid or base used in the anodizing treatment. In addition, oxide films or coating layers produced by the MAO treatment generally have superior properties than those produced by the anodizing treatment. However, because the MAO treatment requires a high operating voltage ranging from 400 V to 500 V, the MAO treatment consumes a significant amount of energy and has a lengthy operating time ranging from approximately 10 minutes to 60 minutes.

Both the anodizing treatment and the MAO treatment require use of harmful substances, consume a lot of energy, and involve complex and lengthy processes, and hence cannot achieve the concept of green development, and the requirements of being environmentally friendly, and having high efficacy.

In view of the aforesaid, there is still a need to develop an effective way for forming a coating layer on a substrate made of a magnesium alloy.

SUMMARY

Therefore, an object of the present disclosure is to provide a method for forming a coating layer on a substrate and a coating layer formed thereby, which can alleviate at least one of the drawbacks of the prior art.

According to one aspect of the present disclosure, the method includes the steps of:

• • (a) providing a substrate which is made of a magnesium alloy;
  • (b) preparing a permanganate conversion solution that contains a permanganate salt, sodium vanadate, and a buffering agent, the permanganate salt being selected from the group consisting of potassium permanganate and sodium permanganate, the buffering agent being selected from the group consisting of monopotassium phosphate, dipotassium phosphate, and a combination thereof;
  • (c) impregnating the substrate of step (a) and an inert conductive material into the permanganate conversion solution of step (b), followed by electrically connecting the substrate to a positive electrode of a power supply such that the substrate serves as an anode, and electrically connecting the inert conductive material to a negative electrode of the power supply such that the inert conductive material serves as a cathode, the inert conductive material being selected from the group consisting of a platinum coated titanium mesh, platinum, a stainless steel, and graphite;
  • (d) applying a voltage ranging from 2 V to 15 V to the substrate in the permanganate conversion solution using the power supply, so as to form a coating layer on the substrate, thereby obtaining a coated substrate; and
  • (e) removing the coated substrate of step (d) from the permanganate conversion solution.

According to another aspect of the present disclosure, the coating layer is formed on a substrate made of a magnesium alloy by the aforesaid method.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the disclosure will become apparent in the following detailed description of the embodiment(s) with reference to the accompanying drawings. It is noted that various features may not be drawn to scale.

FIG. 1 is a schematic flow chart illustrating a method for forming a coating layer on a substrate according to the present disclosure.

FIG. 2 is a schematic view illustrating an experimental apparatus set up in an embodiment of the method of this disclosure.

FIGS. 3 and 4 respectively show corrosion conditions of the control sample 1 and the test sample of EX6 as described in section B of “Property evaluation,” infra.

FIGS. 5 and 6 respectively show corrosion conditions of the control sample 2 and the test sample of EX8 as described in section C of “Property evaluation,” infra.

FIG. 7 is a Nyquist plot showing the electrochemical impedance spectroscopy (EIS) results for the test samples of EX8 and CE1 to CE3 as described in section D of “Property evaluation,” infra.

FIG. 8 is a scanning electron microscope (SEM) image illustrating a surface morphology of the test sample of EX8 as described in section E of “Property evaluation,” infra.

FIG. 9 is a transmission electron microscopy (TEM) image illustrating a cross-sectional view of the test sample of EX8 as described in section E of “Property evaluation,” infra.

DETAILED DESCRIPTION

For the purpose of this specification, it will be clearly understood that the word “comprising” means “including but not limited to”, and that the word “comprises” has a corresponding meaning.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Taiwan or any other country.

Unless otherwise defined, all technical and scientific terms used herein have the meaning commonly understood by a person skilled in the art to which the present disclosure belongs. One skilled in the art will recognize many methods and materials similar or equivalent to those described herein, which could be used in the practice of the present disclosure. Indeed, the present disclosure is in no way limited to the methods and materials described.

Referring to FIG. 1, a method for forming a coating layer on a substrate 120 according to the present disclosure includes steps (a) to (e).

In step (a), a substrate 120 made of a magnesium alloy is provided.

In step (b), a permanganate conversion solution 150 containing a permanganate salt, sodium vanadate (Na₃VO₄), and a buffering agent is prepared. The permanganate salt is selected from the group consisting of potassium permanganate (KMnO₄) and sodium permanganate (NaMnO₄). The buffering agent is selected from the group consisting of monopotassium phosphate (KH₂PO₄), dipotassium phosphate (K₂HPO₄), and a combination thereof.

In step (c), the substrate 120 of step (a) and an inert conductive material 140 are impregnated into the permanganate conversion solution 150 of step (b), followed by electrically connecting the substrate 120 to a positive electrode 132 of a power supply 130 such that the substrate 120 serves as an anode, and electrically connecting the inert conductive material 140 to a negative electrode 134 of the power supply 130 such that the inert conductive material 140 serves as a cathode. The inert conductive material 140 is selected from the group consisting of a platinum coated titanium mesh, platinum, a stainless steel, and graphite.

In step (d), a voltage ranging from 2 V to 15 V is applied to the substrate 120 in the permanganate conversion solution 150 using the power supply 130, so as to form a coating layer on the substrate 120, thereby obtaining a coated substrate.

In step (e), the coated substrate in step (d) is removed from the permanganate conversion solution 150.

<Step (a)>

According to the present disclosure, the substrate 120 may be made of a magnesium-aluminum alloy or a magnesium-lithium alloy. Examples of the magnesium-lithium alloy may include, but are not limited to, a LZ91 magnesium-lithium alloy and a magnesium-lithium-aluminum-zinc alloy. The LZ91 magnesium-lithium alloy contains 89 wt % of magnesium (Mg), 9 wt % of lithium (Li), 1 wt % of zinc (Zn), and 1 wt % of other metal additives. The magnesium-lithium-aluminum-zinc alloy contains approximately 80 wt % of magnesium (Mg), 8 wt % of lithium (Li), 2 wt % to 3 wt % of aluminum (Al), and 1 wt % of zinc (Zn). An example of the magnesium-aluminum alloy may include, but is not limited to, an AZ91D magnesium-aluminum alloy. The AZ91D magnesium-aluminum alloy contains the highest amount of magnesium (Mg), the second highest amount of aluminum (Al) with a content ranging from 8.3 wt % to 9.7 wt %, relative low amounts of zinc (Zn), manganese (Mn), copper (Cu), and silicon (Si), as well as trace amounts of nickel (Ni) and iron (Fe). The AZ91D magnesium-aluminum alloy is widely used in the automotive, aerospace and electronics industries due to its high strength-to-weight ratio and excellent corrosion resistance. The corrosion resistance of the AZ91D magnesium-aluminum alloy can be further enhanced by forming a coating layer according to the method of the present disclosure.

In certain embodiments, the substrate 120 may be subjected to a pretreatment before performing step (b), so as to remove dirt, contaminants, and natural oxides from a surface thereof. Removal of dirt, contaminants, and natural oxides is critical to ensure proper adhesion and performance of the coating layer obtained hereafter. The pretreatment may be selected from the group consisting of alkaline cleaning, pickling, rinsing with deionized water, abrasive polishing, and combinations thereof. To be specific, during the alkaline cleaning, the substrate 120 is impregnated into a mild alkaline solution, so as to dissolve and remove organic contaminants and grease from the surface thereof. During the pickling, the substrate 120 is impregnated into a dilute acid solution, so as to remove contaminants and natural oxides from the surface thereof. During the rinsing, the substrate 120 is rinsed with the deionized water, so as to remove any residual cleaning solution or etching solution from the surface thereof. The abrasive polishing is a mechanical method that utilizes an abrasive material to physically remove contaminants and an oxide layer from the surface of the substrate 120. The aforesaid pretreatments can be conducted separately or in combination according to practical requirements and the type of contaminants present on the surface of the substrate 120.

<Step (b)>

According to the present disclosure, the permanganate salt in the permanganate conversion solution 150 is present in a molar concentration ranging from 0.05 M to 0.20 M. The sodium vanadate in the permanganate conversion solution 150 is present in a molar concentration ranging from 0.005 M to 0.020 M. When the buffering agent in the permanganate conversion solution 150 is monopotassium phosphate, the monopotassium phosphate is present in a molar concentration ranging from 0.005 M to 0.030 M. When the buffering agent in the permanganate conversion solution 150 is dipotassium phosphate, the dipotassium phosphate is present in a molar concentration ranging from 0.005 M to 0.030 M. When the buffering agent in the permanganate conversion solution 150 is a combination of monopotassium phosphate and dipotassium phosphate, the monopotassium phosphate in the combination is present in a molar concentration ranging from 0.005 M to 0.030 M, and the dipotassium phosphate in the combination is present in a molar concentration ranging from 0.005 M to 0.030 M. It is well known in the art that a solution formed by the monopotassium phosphate is slightly acidic and contains dihydrogen phosphate ions (H₂PO₄⁻), while a solution formed by the dipotassium phosphate is weakly alkaline and contains hydrogen phosphate ions (HPO₄^2-). Therefore, the monopotassium phosphate and the dipotassium phosphate can be used as a buffering agent.

According to the present disclosure, the permanganate conversion solution 150 has a pH value ranging from 7 to 12.

<Step (c)>

According to the present disclosure, the substrate 120 of step (a) and the inert conductive material 140 are impregnated into the permanganate conversion solution 150 of step (b), in which the surface of the substrate 120 completely impregnated within the permanganate conversion solution 150 is crucial for forming an uniform coating layer. In addition, the substrate 120 and the inert conductive material 140 are required to be kept at a certain distance apart from each other to ensure that a uniform electric field is formed within the permanganate conversion solution 150. In certain embodiments, the inert conductive material 140 is the platinum coated titanium mesh. The platinum coated titanium mesh, which has platinum (Pt) plated on a surface of titanium (Ti), offers several advantages as the cathode, including high corrosion resistance, excellent electrical conductivity, durability, and inertness.

According to the present disclosure, the substrate 120 and the inert conductive material 140 are impregnated into the permanganate conversion solution 150 at a temperature ranging from 20° C. to 40° C. In order to optimize a deposition of the coating layer obtained hereafter on the surface of the substrate 120, the temperature can be adjusted according to the desired properties of the coating layer.

<Step (d)>

According to the present disclosure, the substrate 120 is applied with a voltage ranging from 2 V to 15 V using the power supply 130 for a time period ranging from 45 seconds to 180 seconds, so as to form the coating layer on the substrate 120. Application of the voltage facilitates an electrochemical reaction that forms the coating layer. In order to optimize a deposition of the coating layer on the surface of the substrate 120, the time period for application of the voltage can be adjusted according to the desired thickness and properties of the coating layer.

<Step (e)>

After removing the coated substrate of step (d) from the permanganate conversion solution 150, the coated substrate is thoroughly rinsed with deionized water to remove any residual solution from the surface thereof, followed by drying at room temperature or with mild heat, so as to ensure that the coating layer formed on the substrate 120 remains intact.

The present disclosure also provides a coating layer which is formed on a substrate 120 made of a magnesium alloy by the aforesaid method of the disclosure.

According to the present disclosure, the method for forming the coating layer on the substrate can reduce energy consumption, decrease generation of harmful substances, reduce operating time, as well as maintain or enhance a performance and durability of the thus obtained coating layer.

The disclosure will be further described by way of the following examples. However, it should be understood that the following examples are solely intended for the purpose of illustration and should not be construed as limiting the disclosure in practice.

EXAMPLES

Example 1 (EX1)

A method for forming a coating layer on a substrate 120 of EX1 includes the following steps (a) to (e).

<Step (a)>

A substrate 120 made of an AZ91D magnesium-aluminum alloy was provided, followed by subjecting the substrate 120 to a pretreatment which includes alkaline cleaning, pickling, and rinsing with deionized water in sequence.

<Step (b)>

A permanganate conversion solution 150, which contained 0.075 M of potassium permanganate (KMnO₄, serving as a permanganate salt), 0.0075 M of sodium vanadate (Na₃VO₄), and 0.0075 M of monopotassium phosphate (KH₂PO₄, serving as a buffering agent) and had a pH value of 10.5, was prepared in a container 110 of an experimental device 100 as shown in FIG. 2.

<Step (c)>

First, the substrate 120 of step (a) and a platinum coated titanium mesh (serving as an inert conductive material 140) were impregnated into the permanganate conversion solution 150 of step (b) at a temperature of 25° C. To be specific, the surface of the substrate 120 was completely impregnated within the permanganate conversion solution 150, and the substrate 120 and the platinum coated titanium mesh were kept at a certain distance apart from each other. Next, the substrate 120 was electrically connected to a positive electrode 132 of a power supply 130 such that the substrate 120 served as an anode, and the platinum coated titanium mesh was electrically connected to a negative electrode 134 of the power supply 130 such that the platinum-coated titanium mesh served as a cathode, as shown in FIG. 2.

<Step (d)>

The substrate 120 was applied with a voltage of 5 V using the power supply 130 for a time period of 60 seconds, so as to form a coating layer on the substrate 120, thereby obtaining a coated substrate.

<Step (e)>

The coated substrate of step (d) was removed from the permanganate conversion solution 150, followed by thoroughly rinsing with deionized water to remove any residual solution from the surface thereof, and then drying at room temperature, such that the coating layer formed on the substrate 120 remained intact. The coated substrate thus obtained served as a test sample of EX1.

The ingredients and the molar concentrations thereof for making the permanganate conversion solution 150, the pH value of the permanganate conversion solution 150, the material of the substrate 120, the operating conditions for performing steps (c) and (d) of the method of EX1 are shown in Table 1 below.

Examples 2 to 7 (EX2 to EX7)

The procedures and conditions in the method for forming the coating layer on the substrate 120 of the respective one of EX2 to EX7 were similar to those of EX1, except that the ingredients and the molar concentrations thereof for making the permanganate conversion solution 150, the pH value of the permanganate conversion solution 150, the operating conditions for performing steps (c) and (d) were varied as shown in Table 1 below. The coated substrates thus obtained served as test samples of EX2 to EX7.

	TABLE 1
	Permanganate conversion			Time period for
	solution	Operating	Operating	application

	Material of	Ingredients and molar	pH	temperature	voltage	of voltage
	substrate	concentrations thereof	value	(° C.)	(V)	(second)

EX1	AZ91D	KMnO4: 0.075M	10.5	25	5	60
	magnesium-	Na3VO4: 0.0075M
	aluminum	KH2PO4: 0.0075M
EX2	alloy	KMnO4: 0.15M	8	25	3	120
		Na3VO4: 0.01M
		KH2PO4: 0.03M
		K2HPO4: 0.005M
EX3		KMnO4: 0.2M	11	25	4	60
		Na3VO4: 0.02M
		KH2PO4: 0.005M
		K2HPO4: 0.005M
EX4		KMnO4: 0.05M	10	40	4	60
		Na3VO4: 0.005M
		K2HPO4: 0.005M
EX5		KMnO4: 0.075M	10	25	5	60
		Na3VO4: 0.01M
		KH2PO4: 0.005M
		K2HPO4: 0.01M
EX6		KMnO4: 0.075M	11	25	4	60
		Na3VO4: 0.015M
		KH2PO4: 0.005M
		K2HPO4: 0.03M
EX7		KMnO4: 0.05M	10	25	15	60
		Na3VO4: 0.005M
		K2HPO4: 0.005M

Example 8 (EX8)

The procedures and conditions in the method for forming the coating layer on the substrate 120 of EX8 were similar to those of EX1, except that the substrate 120 was made of a magnesium-lithium-aluminum-zinc alloy, the permanganate conversion solution 150 contained 0.075 M of potassium permanganate (KMnO₄), 0.0075 M of sodium vanadate (Na₃VO₄), and 0.0075 M of monopotassium phosphate (KH₂PO₄), as well as having a pH value of 10.5, the operating temperature was at room temperature, the operating voltage was 5 V, and the time period for application of the voltage was 60 seconds. The coated substrate thus obtained served as a test sample of EX8.

Comparative Examples 1 to 3 (CE1 to CE3)

The coated substrate serving as a test sample of CE1 was prepared by subjecting a substrate 120 made of a magnesium-lithium-aluminum-zinc alloy to a micro-arc oxidation (MAO) treatment using techniques well-known to those skilled in the art. To be specific, in the MAO treatment, an alkaline electrolyte contained 5 g/L Na₃PO₄·12H₂O and 2 g/L KOH and the substrate 120 was applied with a constant density of 220 mA/cm² for a time period of 10 minutes so as to form a coating layer on the substrate 120.

The coated substrate serving as a test sample of CE2 was prepared by subjecting a substrate 120 made of a magnesium-lithium-aluminum-zinc alloy to an anodizing treatment using techniques well-known to those skilled in the art.

The procedures and conditions in the method for forming the coating layer on the substrate 120 of CE3 were similar to those of EX8, except that in step (c), only the substrate 120 was impregnated in the permanganate conversion solution 150 of step (b) for a certain time period without application of the voltage (i.e., step (d) was omitted). The coated substrate thus obtained served as a test sample of CE3.

Property Evaluation

A. Determination of Corrosion Resistance Grade for Test Samples of EX1 to EX7

Each of the test samples of EX1 to EX7 was subjected to determination of corrosion resistance grade using a neutral salt spray test in accordance with the standard method ASTM D610-08 (published in 2019). To be specific, the test sample was placed in a closed testing chamber at a temperature of 43° C., followed by applying a saline solution with 5 wt % of sodium chloride (NaCl) at a pH value of 7 via a spray nozzle, and then the thus formed saline fog was allowed to settle on the test sample for 120 hours. The degree of the corrosion resistance was assessed visually by scoring on a scale from grade 0 to grade 10. To be specific, grade 2 indicated that a corrosion area was greater than 16% but not greater than 33% of a total surface area of the test sample. Grade 3 indicated that a corrosion area was greater than 10% but not greater than 16% of a total surface area of the test sample. Grade 4 indicated that a corrosion area was greater than 3% but not greater than 10% of a total surface area of the test sample. The results are shown in Table 2 below.

	TABLE 2
	Corrosion resistance grade

	EX1	4
	EX2	4
	EX3	3
	EX4	3
	EX5	3
	EX6	3
	EX7	2

Referring to Table 2, by virtue of adjusting the ingredients and the molar concentrations thereof for making the permanganate conversion solution 150, the pH value of the permanganate conversion solution 150, and the operating conditions for performing steps (c) and (d) of the method of each of EX1 to EX7, the corrosion resistance grade determined in the test sample of each of EX1 to EX7 ranged from grade 2 to grade 4. These results indicate that the coating layer formed by the method of the present disclosure can exhibit good corrosion resistance.

B. Determination of Corrosion Resistance for Test Sample of EX6

A substrate 120 (which was made of the AZ91D magnesium-aluminum alloy and was not subjected to steps (a) to (e) of the method of the present disclosure) (abbreviated as a control sample 1) and another test sample of EX6 were subjected to determination of corrosion resistance using procedures similar to those described in section A of “Property evaluation,” except that the formed saline fog was allowed to settle on the control sample 1 and the test sample of EX6 for 24 hours. The corrosion condition of each sample was visually observed before and after the neutral salt spray test.

FIGS. 3 and 4 show corrosion conditions of the control sample 1 and the test sample of EX6, respectively. As shown in FIG. 3, compared with the control sample 1 before the neutral salt spray test (i.e., the image on the left), a surface of the control sample 1 after the 24-hour neutral salt spray test (i.e., the image on the right) showed severe corrosion, indicating that the substrate 120 made of the AZ91D magnesium-aluminum alloy without the coating layer formed thereon exhibited poor corrosion resistance.

In contrast, as shown in FIG. 4, compared with the test sample of EX6 before the neutral salt spray test (i.e., the image on the left), a surface of the test sample of EX6 after the 120-hour neutral salt spray test (i.e., the image on the right) maintained a significant surface integrity, indicating that the coating layer formed by the method of the present disclosure could effectively prevent the substrate made of the AZ91D magnesium-aluminum alloy from such corrosion even in the harsh environment stimulated by the neutral salt spray test.

C. Determination of Corrosion Resistance for Test Sample of EX8

A substrate 120 (which was made of a magnesium-lithium-aluminum-zinc alloy and was not subjected to steps (a) to (e) of the method of EX8) (abbreviated as a control sample 2) and the test sample of EX8 were subjected to determination of corrosion resistance using procedures similar to those described in section A of “Property evaluation,” except that the formed saline fog was allowed to settle on the control sample 2 for 8 hours. The corrosion condition of each sample was visually observed before and after the neutral salt spray test.

FIGS. 5 and 6 show corrosion conditions of the control sample 2 and the test sample of EX8, respectively. As shown in FIG. 5, compared with the control sample 2 before the neutral salt spray test (i.e., the image on the left), a surface of the control sample 2 after only 8 hours of the neutral salt spray test (i.e., the image on the right) showed severe corrosion, indicating that the substrate 120 made of the magnesium-lithium-aluminum-zinc alloy without the coating layer formed thereon exhibited poor corrosion resistance.

In contrast, as shown in FIG. 6, compared with the test sample of EX8 before the neutral salt spray test (i.e., the image on the left), a surface of the test sample of EX8 after the 120-hour neutral salt spray test (i.e., the image on the right) showed a significantly enhanced corrosion resistance and had a corrosion area that was less than 5% of a total surface area of the test sample of EX8, indicating that the coating layer formed by the method of the present disclosure could effectively prevent the substrate 120 made of the magnesium-lithium-aluminum-zinc alloy from such corrosion even in the harsh environment stimulated by the neutral salt spray test. These results demonstrate that the method of the present disclosure can be applied to substrates made of different materials.

D. Electrochemical Impedance Spectroscopy (EIS) Analysis

Each of the test samples of EX8 and CE1 to CE3 and control sample 2 was placed in a saline solution with 3.5 wt % of sodium chloride (NaCl), followed by subjecting the respective sample to electrochemical impedance spectroscopy (EIS) analysis using techniques well-known to those skilled in the arts, so as to obtain a Nyquist plot to compare corrosion resistance of the respective sample.

FIG. 7 is a Nyquist plot showing the EIS results for the test samples of EX8 and CE1 to CE3. As shown in FIG. 7, the corrosion resistance of the test sample of EX8 was superior to those of the test samples of CE2 and CE3, but inferior to that of the test sample of CE1. Although the corrosion resistance of the test sample of CE1 was superior to that of the test sample of EX8, the MAO treatment for preparing the test sample of CE1 required a significant amount of energy (i.e., a high operating voltage ranging from 400 V to 500 V) and a lengthy operating time (i.e., a time period for application of voltage ranging from approximately 10 minutes to 60 minutes). In contrast, the method for preparing the test sample of EX8 required less energy (i.e., an operating voltage ranging from 2 V to 15 V) and a shorter operating time (i.e., a time period for application of voltage ranging from 45 seconds to 180 seconds). These results demonstrate that the method of the present disclosure is capable of forming the coating layer with excellent corrosion resistance under low-coat operating conditions.

E. Morphological Observation for Test Sample of EX8

The test sample of EX8 was subjected to scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis using techniques well-known to those skilled in the art. The thus obtained SEM image showing a surface morphology of a coating layer of the test sample of EX8 was shown in FIG. 8, and the thus obtained TEM image showing a cross-sectional view of a coating layer of the test sample of EX8 was shown in FIG. 9.

As shown in FIG. 8, a surface of the coating layer of the test sample of EX8 had no significant cracks. As shown in FIG. 9, the coating layer of the test sample of EX8 had a dense structure and a thickness of approximately 2 μm. These results indicate that the coating layer formed on the substrate 120 made of the magnesium alloy according to the method of the present disclosure exhibits good corrosion resistance.

Summarizing the above test results, it is clear that the method for forming the coating layer on the substrate 120 of the present disclosure can effectively reduce energy consumption, reduce operating time, decrease generation of harmful substances by having the specific ingredients in the permanganate conversion solution 150, thereby ensuring that the coating layer formed on the substrate 120 has an excellent quality.

In the description above, for the purposes of explanation, numerous specific details have been set forth in order to provide a thorough understanding of the embodiment(s). It will be apparent, however, to one skilled in the art, that one or more other embodiments may be practiced without some of these specific details. It should also be appreciated that reference throughout this specification to “one embodiment,” “an embodiment,” an embodiment with an indication of an ordinal number and so forth means that a particular feature, structure, or characteristic may be included in the practice of the disclosure. It should be further appreciated that in the description, various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of various inventive aspects; such does not mean that every one of these features needs to be practiced with the presence of all the other features. In other words, in any described embodiment, when implementation of one or more features or specific details does not affect implementation of another one or more features or specific details, the one or more features may be singled out and practiced alone without the another one or more features or specific details. It should be further noted that one or more features or specific details from one embodiment may be practiced together with one or more features or specific details from another embodiment, where appropriate, in the practice of the disclosure.

While the disclosure has been described in connection with what is (are) considered the exemplary embodiment(s), it is understood that this disclosure is not limited to the disclosed embodiment(s) but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.