Although certain anodizing processes simultaneously remove contamination from and produce anodic films on magnesium alloy surface, generally the chemical and anodic treatments require the article surface to be free from all contaminations and impurities. Appropriate cleaning methods can effectively remove impurities on the surface, such as oxide rust, grease, rolling scale, silicate sands, as well as other metal debris or intermetallic compounds. Failure of removing these contaminations, particularly heavy metal impurities, will lead to galvanic corrosion and coating delamination, hence the reduced protective value. Therefore, cleaning process is crucial to obtain good corrosion protection. For magnesium alloys, the commonly used cleaning processes include mechanical pretreatment, solvent cleaning or alkaline cleaning, and acid pickling or an etching step (Avedesian and Baker 1999; Kainer and Kaiser 2003; Friedrich and Mordike 2006).

Mechanical Cleaning

Surface cleaning by mechanical methods has been around for almost all types of metal. Mechanical cleaning is usually taken as a preliminary step on raw magnesium alloys. According to the surface quality and geometry of the magnesium alloy articles, appropriate mechanical treatments include blasting, grinding, and polishing. These mechanical cleaning methods can effectively remove majority of contaminants on article surface; nevertheless, some metal loss is involved during these processes, which could affect the surface quality and geometry of precise die casting parts.
It has to be noted that mechanical cleaning in dry conditions should be avoided whenever possible. Magnesium debris is flammable, for example, the mixture of magnesium dust with air at a certain ratio is highly explosive; burning of larger debris particles can be causes of fire and other accidents (Kainer and Kaiser 2003; Herbert 2011; Dickman 2011; ASM International 1996; Kostilnik 2005).

Sandblasting

Sandblasting is operated under wet condition, known as air blast and wet blast (Kainer and Kaiser 2003; Hillis 2005; Herbert 2011; Kostilnik 2005). Sandblasting under dry condition is reported, but not recommended. Purified sand (SiO2) is typical for blasting magnesium alloy surface; other sands such as glass beads and corundum (Al2O3) are also used. It should be noted that the blasting sands must be free of any metal contamination, particularly iron, copper, and other heavy metals, as these impurities can cause galvanic corrosion on the magnesium surface and reduce the resistance against corrosion dramatically. Therefore, the use of metal grit is not recommended. On the other hand, magnesium alloy materials are relatively soft than SiO2 or Al2O3; the blasting pressure needs to be adjusted to avoid sand embedment into the material surface.

Grinding

Grinding is commonly used on magnesium alloys for the removal of flash and surface imperfections from die castings or scratches from surfaces of extrusions (Kainer and Kaiser 2003; Hillis 2005; ASM International 1996; Kostilnik 2005). Typical speed of grinding on belts or disks is in the range from 900 to 1,800 m/min. Silicon carbide and corundum at 60–400 mesh are recommended for use as grinding agents. Grinding magnesium alloys at dry conditions should be avoided, because the mixture of magnesium debris with air at a certain ratio is highly explosive. Magnesium debris or dust must not be allowed to accumulate, because any accidental sparks produced from contacting between the grinding disk and metal parts or other components of the grinding machine can lead to explosion. For this reason, wetting and dust suction device are required for grinding magnesium alloys.

Polishing

Magnesium alloys are also suitable for polishing. Cotton-covered polishing disk is recommended. Typical speed of polishing is in the range from 1,800 to 2,500 m/min (Kainer and Kaiser 2003; Hillis 2005; Dickman 2011; Kostilnik 2005). However, polishing of magnesium alloys is usually not necessary in standard practices for preparation of magnesium alloy surfaces for moderate to severe environment, as the subsequent chemical treatment and anodic treatment rendered will roughen the surface again (ASTM D1732 2007; ASTM B879 2003).

Chemical Cleaning

After the preliminary mechanical cleaning, some impurities may still remain on magnesium alloy surfaces. These contaminations can be removed by acid pickling or fluoride anodizing. In other cases, the raw magnesium alloy materials are often received with temporary protective grease or wax layer over the surface, very similar to the case of cold roll steel. Therefore, alkaline or solvent degreasing is required to expose the metal prior to acid pickling or fluoride anodizing. In addition, magnesium alloy articles that have been mechanically treated are also recommended to go through a degrease process to avoid any leftover machine oil.
The following standards cover processes which are needed for the preparation of metal surfaces prior to the application of surface pretreatments and coatings:

ISO 27831-2:2008 Metallic and other inorganic coatings – Cleaning and preparation of metal surfaces – Part 2: Non-ferrous metals and alloys (ISO 27831-2:2008)
ASTM D1732 Standard Practices for Preparation of Magnesium Alloy Surfaces for Painting (ASTM D1732 2007)
ASTM B879 Standard Practices for Applying Non-Electrolytic Conversion Coatings on Magnesium and Magnesium Alloys (ASTM B879 2003)
ASTM B322 Practice for Cleaning Metals Prior to Electroplating (ASTM B322 2003)

Alkaline Degrease

Magnesium alloys have a good resistance to common alkalis except pyrophosphates and some polyphosphates. The level of metal removal in common alkaline degreaser above a pH of 12 is negligible. Typically, alkaline degreaser for low-carbon steel can effectively remove the oil or grease present on the magnesium surface without causing any significant corrosion (see Table 3) (Avedesian and Baker 1999; Kainer and Kaiser 2003; Friedrich and Mordike 2006; Hillis 2005; Cormier 2005). Alkaline degreaser containing soap-like surfactants, such as octylphenol ethoxylates, nonylphenol ethoxylates, alcohol ethoxylates, secondary alcohol ethoxylates, and alkylamine ethoxylates, is recommended.
The alkaline degreasing process can also be accelerated by applying a moderate cathodic potential on the article, known as cathodic cleaning. This electro-assisted method generates hydrogen on the magnesium alloys and promotes the removal efficiency of the impurities on the surface. The current density on the magnesium alloy articles is recommended to be controlled within 5 A/dm2, and normally, the potential required to sustain the current is approximately - 4 to - 8 V (Friedrich and Mordike 2006; Hillis 2005). By means of cathodic cleaning, the entire degreasing process can be effectively reduced to 3–5 min. It should be noted that the corresponding anode electrode has to be carefully chosen, because any heavy metal ions that dissolve from the anode would eventually deposit on the magnesium alloy surface, resulting in galvanic problems.

Solvent Degrease

Organic solvents are also used to remove grease, oil contaminants, and protective wax. Normally, the solvent degrease process involves both solvent immersion and solvent vapor treating; the latter is required for completely cleaning the magnesium alloy surface (Avedesian and Baker 1999; Friedrich and Mordike 2006; Hillis 2005; Rupp and Surprenant 2005). The combination of immersion and vapor treating can effectively clean the surface without any metal removal, as shown in Table 4. Trichloroethylene and perchloroethylene are the most often used, and methylene chloride is effective in removing the excess organic resin. Methanol should not be used, as it may react with magnesium surfaces.

Acid Pickling

Acid pickling is suitable for removing contamination that is tightly bonded to the surface or insoluble in alkalis or solvents (Avedesian and Baker 1999; Friedrich and Mordike 2006; Hillis 2005; Hudson 2005). However, acid pickling should be applied to magnesium alloys carefully, as the alloy surface can be easily roughened and part of the metal would dissolve into the solution. Usually based upon the concern of etching effect, acid pickling is limited to die casting parts or those with a rough surface. On the other hand, acid pickling is effective on removing heavy metal impurities, as well as rust and residual conversion coatings on reused magnesium alloys.
As shown in Table 5, most pickling agents contain chromic acid and hydrofluoric acid, which may increase the surface porosity, as different phases on the magnesium alloy surface dissolve at different rates. The associated corrosion resistance and mechanical strength may be affected. For this reason, the use of acid pickling should be carefully considered.

Fluoride Anodizing

Fluoride anodizing is applicable to all magnesium alloys and is recommended for all magnesium alloy articles prior to protective pretreatments and finishing (Table 6). Compared to acid pickling, fluoride anodizing is safer and more effective on removing contaminants of heavy metal left from mechanical cleaning, such as metallic debris from grinding or blasting (Friedrich and Mordike 2006; Hillis 2005).
During fluoride anodizing, an AC voltage is applied and gradually increased until 90–120 V. Intensive current is generated at first but diminishes rapidly as impurities are removed and a barrier layer of MgF2 forms on the article surface. This process is complete when the current drops to blow 0.5 A/dm2, usually within 10–15 min. The fluoride film produced during the process is a weak base for adhesion and needs to be removed in chromic acid rinsing followed by hydrofluoric acid rinsing before chemical conversion coatings, but not necessary if the article will be subsequently subjected to anodizing.

Chemical Conversion Coating

Chemical conversion coatings are a family of chemically bonded superficial layer used to replace the native oxide surface and to passivate metal substrates. They are formed by contacting a metallic surface with an aqueous solution, which conventionally contains film-forming agents such as soluble chromate species. Since magnesium is highly reactive, once exposed to air, the surface of magnesium alloys is spontaneously oxidized, forming a thin oxide layer. However, the passivity of the thin oxide layer is low, which only provides limited protection in atmospheric exposure. Chemical conversion coating can be applied on magnesium alloys to serve as a better passive layer. In contrast to the native magnesium oxide, chemical conversion coatings have dense structures and improved corrosion resistance owning to their inert chemical composition, typically trivalent chromium oxide, or phosphate. Meanwhile, chemical conversion coatings play an important role in improving the adhesion of organic primers and finishing paintings.
The following standards cover processes which are needed for the application of chemical conversion coatings:

ASTM D1732 Standard Practices for Preparation of Magnesium Alloy Surfaces for Painting (ASTM D1732 2007)
ASTM B879 Standard Practices for Applying Non-Electrolytic Conversion Coatings on Magnesium and Magnesium Alloys (ASTM B879 2003)

Principles of Chemical Conversion Coatings

Chemical conversion coatings provide magnesium alloys with a passive surface against corrosion and a base layer for further polymer paint (Chen et al. 2011a; Ono et al. 1998, 2001; Chong and Shih 2003; Wu et al. 2005; Weng et al. 2006; Zheng and Liang 2007; Takai and Takaya 2008; Umehara et al. 2003; Simaranov et al. 1989, 1992). Chemical conversion coatings are formed through a series of spontaneous chemical reactions, which can be generalized into two stages (Simaranov et al. 1989, 1992). During the first immersion stage, the native magnesium oxide and part of the magnesium substrate dissolve into the solution, as the conversion coating bath usually has a low pH value. Meanwhile, the conversion coating species precipitates and deposits onto the substrate surface, forming a gel-like hydroxide layer. In the second stage, the hydroxide layer further undergoes dehydration and condensation. Eventually, a dense barrier coating is formed over the magnesium alloy substrate with a rough and dendrite-like surface. The actual chemical conversion coating formation mechanism on magnesium alloy is more complicated and different between each different type; however, the simplified model is widely accepted for understanding the coating structure and the chemical treatment process.
Although different versions of chemical conversion coating systems may contain different compounds such as chromates and phosphates, they are commonly characterized by acting as a passive layer that replaces the original magnesium oxide protecting the metal substrate. The new conversion coating layer is thicker, denser, more chemically stable, and with less permeability to aqueous environment, some even containing corrosion-inhibiting compounds as a reservoir of self-healing agents.
Chemical conversion coatings are also known as a good base layer for bonding organic primer and finishing paintings owning to their rough and dendrite-like surface. It should be noted that the corrosion protection by a chemical conversion coating layer itself is only temporary and not considered to be protective finishes. These coatings must be used in conjunction with other primers or surface finishing coating to exhibit full potential.

Chromate Conversion Coatings

Chromate conversion coatings have been shown to significantly improve the corrosion resistance on all types of magnesium alloys in moderate service conditions. Substrates with fine-grained microstructure best respond to chromate-based conversion coatings. The first type of commercial chromate conversion coatings for magnesium alloys was known as Dow 1 (ASTM D1732 2007; ASTM B879 2003; Simaranov et al. 1989, 1992). Because of the low cost and good performance, this type of conversion coating has been used for decades. However, the critical disadvantage of chromate conversion coatings is the content of various hexavalent chromium compounds, which are toxic and carcinogenic, leading to increasingly restricted usage.
The chromate conversion coating is dominated by Cr2O3, which is both chemically inert and physically robust; in addition, the coating also contains some Cr (VI) species remained from the solution as well as some magnesium compounds such as MgO2, MgF2, and NaMgF3, among which Cr(VI) species are still chemically active, and provides the treated magnesium surface with self-healing capability over long-term usage.
Some other commercial chromate conversion coating solutions contain HF, which is known as a surface etch agent or activator (ASTM D1732 2007; ASTM B879 2003). HF is added to assist the removal of the native magnesium oxide and other metal impurities on the substrate surface. Sometimes, HF activation is separated from the conversion coating bath as a pre-step prior to the coating deposition. Generally, the addition of HF can improve the efficiency of the coating deposition and the quality of the coating structure.

Dow 1

Dow 1 conversion coating system, also known as acid chromate treatment or chrome pickle developed by Dow Chemical, was the first generation of commercialized chromate conversion coating systems for magnesium alloys. In Table 7, a significant content of HNO3 is present in the immersion bath, which is designed to sustain the pH and strip off the native oxide surface. However, the magnesium alloy substrate dissolves rapidly in Dow 1, which may cause approximately 15–25 μm loss by surface dissolution. Therefore, the Dow 1 treating process is restricted within 2 min and not recommended for finely machined parts or articles with tight tolerances. Table 7 gives the standard practices of Dow 1 according to ASTM B879 and D1732 (ASTM D1732 2007; ASTM B879 2003).

Dow 7

In Dow 7, or known as dichromate treatment, the HNO3 is removed, and an independent HF activation pre-step is added prior to the immersion (Table 8). A thorough rinse is required after HF activation, as excessive F- residuals may reduce the quality of the final conversion coating, leading to lower corrosion resistance. Compared to Dow 1, Dow 7 treatment only causes negligible metal loss on magnesium alloy surface and produces film that is excellent for paint base.

Chrome Manganese and R.A.E. Bath

These later developed chromate-based conversion coating systems have a reduced content of chromate and amoderate pH value.Metal loss due to etching or dissolution during the immersion stage is negligible (4–6). Table 9 gives the chemical composition and condition for chrome manganese coating and R.A.E. bath.

Chromic Acid Brush-On

Repairing damaged surface on the chromate-treated magnesium alloy articles emphasizes an easy but effective method. For this reason, using a simplified but similar performed method is much more practical than re-treating the damaged articles in the original immersion bath. These simplified systems are designed to fix local damages by wiping or swabbing, known as touch-up chromate conversion coatings. The rest part of the article surface remains intact. Table 10 gives some commercial touch-up products developed by Dow and Henkel.

Chromate-Free Conversion Coatings

Recent research has focused on the development of various non-chromate and chromium-free systems in response to environmental protection laws, chromate minimization policies, and other regulations (Fahrenholtz et al. 2002; Kendig and Jeanjaquet 2002; Zhang et al. 2005; Yong et al. 2008; Costa 1997; Costa and Klein 2006; Hu et al. 2009). Nevertheless, the performance of non-chromate systems is still not comparable to chromate-based systems particularly in severe corrosion environment.
Generally, these non-chromate replacements rely on phosphate and compounds containing transition-metal elements such as Mn, V, Zr, and Mo to serve as the main material source of forming the inert barrier layer. Similar to Cr, these elements possess multilevel valency stages: (1) at high valency stage, they are very strong oxidants, like chromate, and can oxidize magnesium by reducing to a more stable state, and (2) at low valency stage, they are less soluble and chemically stable. From this point of view, these non-chromate conversion coating systems work in a very similar way that the chromate system does on magnesium alloys. The current development of non-chromate replacement has identified some promising systems that provide comparable corrosion protection in moderate corrosion environment; however, the overall performance still needs improvement, such as deposition rate, coating firmness, solution stability long-term performance, ease of applying, etc.

Phosphate-Permanganate Conversion Coatings

The concept of using permanganate-based system as a conversion coating treatment for magnesium alloys is based on its similar chemical properties to chromate (Umehara et al. 2001, 2003; Zucchi et al. 2007). However, the corrosion protection performance solely based on permanganate treatment is not satisfactory in practical application. It has been found that the Mn(IV) species precipitate too fast, resulting in a loose structure rather than a dense conversion coating layer. To overcome this problem, permanganate is used in combination with phosphate. The phosphate-permanganate system works successfully on magnesium alloys (Table 11) (Chen et al. 2011a). In addition, it is more environmentally friendly and has been shown to have corrosion resistance comparable to chromate treatments.

Fluorozirconate Conversion Coatings

Fluorozirconate-based treatments have also shown to be a promising alternative for magnesium alloy articles (Chen et al. 2011a; Tomlinson 1995, 1988, 1999). Zirconium is believed to form continuous three-dimensional polymeric or metalloidoxide matrix in a similar way as chromium (Atanasyants and Nikitin 2001; Mogoda 1999). This makes zirconate-based system an attractive choice for non-chromate alternatives. However, disadvantages of this type of treatment are the low stability with hard water and the relatively weak corrosion resistance in severe corrosion environment. Recent efforts on improving fluorozirconate-based conversion coatings have been focused on systems that combine other components such as vanadate, chromium(III), and cerium(IV) (Zanotto et al. 2011; Wang et al. 2009; Lin and Fang 2005; Li et al. 2008; Ardelean et al. 2008).