Conversion Coating

Magnesium and its alloys have many physical and mechanical properties that make them useful in number of applications. Properties such as high strength/ weight ratio make it an ideal metal in many applications. Unfortunately, magnesium and its alloys are highly susceptible to corrosion, which has impacted their use in many industries. The simplest way to lessen the effects of corrosion is by preventing its direct contact with the environment. Many methods are used to mitigate these effects but the two most prominent ones are conversion coating and electrophoretic coating (E-coating).

Conversion coating

Chemical conversion solutions could consist of few simple materials or many chemical compounds. A typical conversion solution contains the following: oxidants, promoters, corrosion inhibitors, wetting agents and pH buffer regulators. Oxidants are primarily used to speed up the cathodic reaction (kinetics). They consume a lot of H^(+), raise the OH^(-) concentration as well as speed up the dissolution of magnesium and the formation of conversion film. Some common oxidants are nitrae and perchlorate. The main function of promoters is to initially precipitate on the surface of Magnesium alloys to remove the conversion layer. Some common examples of promoters are zirconium and vanadic salts. The purpose of corrosion inhibitors is to be adsorbed on the surface of magnesium alloys to reduce the reaction rate of the dissolution of Mg2+. In addition, they also stabilize the solution by forming complexe with Mg2+ and also control the conversion layer formation rate. Wetting agents are primarily there to reduce the surface tension of magnesium alloys and make the precipitation of conversion layer much easier. Furthermore, they also improve the adhesion of the conversion layer. An example of this includes sodium dodecyl benzene sulfonate. Finally, pH buffer regualtors are used to control the pH of solutions which impacts the speed and quality of the conversion layer. High pH increases the kinetics of the deposition of conversion layer but causes the poor adhesion and softness of the conversion layer.

Chemical conversion processes currently used for magnesium and its alloys can be classified into the following:

·  Chromate conversion coatings

·  Stannate conversion coatings

·  Rare earth (e.g. cerium, lanthanum and praseodymium) oxide conversion coatings

·  Phosphate, phosphate-permanganate and fluoride-related conversion coatings

·  Conversion coatings based on compounds of V, Zr, Mo, W, Ti, Co.

Most of the chemical conversions typically use chromate and phosphate conversions; therefore, they will be described in more detail.

Chromate conversion coating

Chromate conversion treatment is a very fast process, usually takes 30-60 sec and can operate at room temperature. Chromate conversion solution consists of chromic acid (H2Cr04), chromate salts and certain activator ions such as sulfates, chlorides, fluorides, phosphates and complex cyanides with pH around 1-2. As seen, the chromate conversion treatment process is acidic, which causes the dissolution of magnesium (Mg) into the solution as Mg2+. This increases the pH of the metal-solution interface. Then, Mg2+ ions combine with chromate ions to form a compound that is insoluble at the locally higher pH region. This mixture of compound precipitates on the metal surface and causes an adherent coating.

Commercially available chromate conversion solutions such as DOW and JIS have been widely applied to improve the corrosion resistance of magnesium alloys. Chromate conversion treatment is a very reliable process that provides excellent corrosion protection for many metals. A major drawback is the environmental impact of this type of conversion, which in recent years have limited its applications. Having said that, there are still efforts in developing low chromic content conversion solutions to treat magnesium alloys to minimize the environmental impact.

Phosphate and phosphate-permanganate conversion coating

Phosphate-permanganate conversion solution typically consists of KMnO4, MnHPO4, K2HPO4, and H3PO4 with a solution pH of around 3-6. The process typically involves magnesium to be placed in the phosphoric acid, which reduces the H+ ions, thus raising the pH and causing the dissolved Mg2+ to fall out of the solution and precipitate on the surface as Mg2(PO4)3. Permanganate ions (Mn04) that also exist in the solution are reduced from Mn7+ to form manganese oxides on the surface of the Mg samples. This can be explained using the reaction below:

MnO, + 4H+ + 3e- + Mn02(s) + 2H20

or

2Mn02 + 2H+ + 2e- + Mn2O3 + H20

or

MnO, + 2H20 + 3e- + Mn02(s) + 40H

As the OH- concentration increases at the anode site, a raise in pH value is observed. This causes the formation of manganese products such as MnO2, Mn2O3, Mn3O4 and MnOOH and they co-exist within the conversion coating layer. This method is found to be very comparable to that of chromate conversion treatment.

Electrophoretic coating (E- COATING)

Electrophoretic coating is a process that colloidal particles suspended in a liquid medium migrate under the influence of anelectric field and are deposited onto an electrode. Electro-coating can be divided into anodic and cathodic processes. Anodic process involves a film of oxide produced on a metal by electrolysis with the metal as the anode. The deposition of the negatively charged particles in the electrolyte is driven by and electrical field. In the cathodic electro-coating process, the work piece that acts as a cathode attracts positively charged particles under and electrical field. There are several advantages of electro-coating process. E-coating produces equal coating with low porosity providing corrosion protection. Coating of complicated shaped surfaces is also possible with high throwing power though this process. E-coat is inexpensive for mass production. This water based process has low level of volatile organic compounds with a minimal waste discharge in the E-coat plant.

Surface conditions are important for deposition, bonding strength and durability of the process ensuring the quality of the coatings. Many different treatment methods of the substrates have been discovered to modify the surface conditions, with the aim of removing the contaminations such as grease, oil, silicone and dirt that have adverse effect on the coating that changes the surface appearance. Alkaline solution treatment is the most common process used for cleaning the metallic substrates before electro-coating. Hsu explored wet class as a surface preparing method that can remove rust and contaminants from the surface of magnesium alloys. Fire hazard of particles is low because Mg particles oxidize in the liquid.

The two important coating processes that are involved in forming a base coating layer for electro-coating are conversion and phosphate coating. Chromate conversion coating that involve toxic Cr6 ions is a conventional way of generating base coatings on magnesium and Mg alloys. Chrome free conversion methods such as phosphate coating and permanganate coating are being introduced to replace the original chromate coating process. Conversion coating containing chrome can be replaced by silica-modified phosphates. Adding rare Earth elements such as cerium to the phosphate layer can minimize the cracking of the layer and increases stability leading to corrosion resistance and coating durability. Adding molybdate to the phosphate coating both results in the formation of a conversion coating containing nanocrystalline zinc phosphate and MgMo04.

The process of oxidization forms a layer of Mg oxide before electro-coating that increases corrosion resistance. The corrosion resistance has been improved because of the layer of oxide formed by anodizing. Plasma treatment of Mg improves the bonding strength of the coating. Cathodic coating as revealed by the N-methylpyrrolidone (NMF') test increased from less than 1 hour to over 22 hours by appropriate plasma treatment of the magnesium surface. The oxide layer produced by micro-arc oxidation or anodizing has pores that can form interlocks with the paint deposited by subsequent electro-coating.

Initial bonding strength of the polymer to metal substrates depends on the nature of the oxide on the surface. The long-term durability of the coatings depends on the environmental stability of the underlying oxide.

The process of deposition involves the application of a DC voltage across the working electrode. Electrolysis accompanies the charged particles that bath towards oppositely charged electrodes. The water electrolysis will result in oxygen gas liberation on the anode and hydrogen gas evolution on the cathode. This process results in the liberation of oxygen on the anode and hydrogen on cathode. The equal distribution of these gasses surrounding the electrodes lead to a pH change will in turn de-stabilize the paint components of the solution. The factors that could affect this process include the chemistry of the coating bath and the process parameters such as applied voltage, coating time and bath temperature.

There are two types of electrolysis. Non-aqueous electrophoretic deposition have been used in the fabrication of electronics and production of ceramic coating. The aqueous-based electrolyte consists of an emulsion of polymer and deionized water in a stable condition. The deionized water acts as the carrier of the paint solids including resin, pigment and solvent. Resin is the backbone of the final paint film and provides protection of the substrate from various damages. pigment provides color and gloss of the coating. solvent improves the smoothness and appearance of the coating film. There are two types of chemistries for the coating materials. Epoxy-based is more corrosion resistant than acrylic-based coating which is more resistant to ultraviolet radiation and is based on free radical initiated polymers containing monomers.

The most important variable in the process of electro-coating process is the voltage. Higher voltage provides higher throwing power for the process but can lead to the rupture of the coating resulting in porous appearance. The voltage used in the industry is the range of 200-300V. Higher voltage lead to thicker coating film. The required voltage in the cathode process is higher than in the anode process. Timing of the coating is also very crucial. The initial coating current is high that then drops and that's why the time for the process is short. The bath temperature is important as the conductivity of the bath and the deposited coatings both increase with increase in the bath temperature. The increased bath conductivity will lead to decreased ‘rupture’ voltage. Viscosity of the coating film changes with the bath temperature.

Electroless E-coating of Mg alloys is an interesting electro-coating development. In this process, pre-treated Mg alloy is immersed in the electro-coating bath for a couple of seconds without applying the electrical field and then is pulled out and dry. A thin layer of stable paint film can be formed on the surface of a various Mg alloys, even after the surface is anodized that improves the corrosion resistance of the magnesium alloys.

After deposition, the coated part is normally rinsed to clean the surface. Rinsing with water takes place and the water is then returned to the coating bath allowing for a high utilization efficiency of the coating materials and reducing the amount of waste discharged into the environment. The coated parts are placed in the oven after rinsing to cure the coated film. In baking process, the polymer undergoes cross linking and becomes hard and resistant to chemical attack. It also allows the coating to flow out to fill the gas pores formed during the deposition process to make the coating film dense and continuous. The temperature for baking is in the range of 82-177°C (180-350°F).

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