Influence of pH of Phosphating Bath on the Zinc Phosphate Coating on AZ91D Magnesium Alloy

Abstract

Magnesium alloy has some advantageous properties including low density and high strength to weight ratio, [1] high thermal conductivity, good electromagnetic features and being easily recycled. These properties make it valuable in a number of industrial fields including automobile, aerospace components, mobile phones and sporting goods. [2][3][4][5][6][7] For example, the use of magnesium alloys can significantly decrease the weight of automobiles without sacrificing structural strength.When magnesium alloys are exposed to the atmosphere, their surfaces form a kind of film consisting magnesium oxide and magnesium hydroxide that gives limited protection from further attack. The film has no the same self-healing property as the film of aluminum alloy that formed in atmospheres. In reverse, chlorides, sulfates, or other hydrophilic substances promote corrosion by destroying this film easily. Unfortunately, this poor corrosion resistance of magnesium alloys has hindered their widely applications where exposure to rigorous service conditions is unavoidable.More interest was paid to the investigations of corrosion of magnesium alloy in recent years. [8][9][10][11] Anticorrosion properties of magnesium alloy can be improved by application high-purity alloys and surface protection. Producers of magnesium have demonstrated the importance of high-purity alloys for structural applications. However, surface contamination from handling and mechanical treatment can greatly degrade the corrosion resistance of high-purity alloys. One of the most effective ways to prevent corrosion is to coat the base materials. Some surface protection reports of magnesium alloy were published including electroplating Ni-P, [12] chromate conversion coating, [13] Zn-Ni alloy coating, [14] anodizing film, [15,16] and plasma surface treatment film. [17] Among these possible treatment, zinc phosphatization may be one of the most promising methods for magnesium alloy anticorrosion and pretreatment process before paint. Especially, it has the potential to replace the conventional chromate treatment where the environment polluted hexad chromium was involved. There were limit reports of phosphate coatings on magnesium alloys. Phosphate films were gained from phosphate-permanganate bath, which have micro cracks, [18][19][20] similar microstructure to chromate conversion. Recently, a zinc phosphate (Zn 3 (PO 4 ) 2 ·4H 2 O) coating was obtained on AM 60 magnesium alloy. [21] Han et al. obtained a phosphate film of Mn 3 (PO 4 ) 2 on AZ31D magnesium alloy in a bath containing phosphate and manganese. [22] In a previous work, a phosphate coating with the compositions in the coating being Zn 3 (PO 4 ) 2 ·4H 2 O, metal Zn and AlPO 4 was obtained. [23] In fact, phosphatization of magnesium alloy is very different and rather difficult in comparison with the traditional phosphatization of steel and aluminum because of high electrochemically activity of magnesium. In the acid phosphating bath resolving rate of magnesium must be restrained in order to obtain dense a...

Anticorrosive interfacial coatings for metallic substrates

“…In a systematic study by Li et al, 56 the effect of the pH of the bath on zinc-phosphated magnesium was investigated. The pH was changed over a wide range, and it was found that lower pH values (between 2·05 and 2·5) significantly improved the coating microstructure from flake-cluster morphology to slab-like feature indicating a more coverage on the surface (Figure 4).…”

Phosphate chemical conversion coatings on metallic substrates for biomedical application: A review

“…The concentration of reactants, especially accelerators used in the phosphating bath, affects the crystal size and morphology of the coating to some extent [27]. The pH of phosphating bath has a strong affect on characteristics of the coating, too [31]. It is vital in determining the rate and the amount of coating formed [32].…”

Surface modification of magnesium and its alloys for biomedical applications

“…Deposition of phosphate conversion coatings has been explored by many researchers as a surface modification method to improve the corrosion resistance of Mg and its alloys (Hu, Meng, Chen, & Wang, 2012;Li, Lian, Niu, & Jiang, 2006; Van Phuong, Lee, Chang, Kim et al, 2013;Xu, Zhang, & Yang, 2009). Deposition of a Zn phosphate coating is one of the most well-studied systems and was recently reviewed by Van Phuong, Lee, Chang, Kim et al (2013).…”

“…Deposition of a Zn phosphate coating is one of the most well-studied systems and was recently reviewed by Van Phuong, Lee, Chang, Kim et al (2013). Being a conversion coating, the resultant phosphate coating is porous in nature, which limits its fullest potential in improving the corrosion resistance of Mg alloys (Hu et al, 2012;Li et al, 2006 (Chen, Birbilis, & Abbott, 2011;Wang, Huang et al, 2013). Being a conversion coating, the resultant phosphate coating is porous in nature, which limits its fullest potential in improving the corrosion resistance of Mg alloys (Hu et al, 2012;Li et al, 2006 (Chen, Birbilis, & Abbott, 2011;Wang, Huang et al, 2013).…”