Forming of high corrosion and abrasion resistant surface films using magnesium oxide on magnesium alloys

发展了三种镁及镁合金表面改性新技术。在第一种技术中，注意到镁合金在腐蚀反应中生成氢氧化镁。在腐蚀反应的早期阶段，氢氧化物均匀地形成，覆盖在腐蚀表面，表现出相对较弱的耐腐蚀性。但氢氧化膜不能抑制后续的腐蚀反应，呈丝状腐蚀。简单地看一下氢氧化镁的化学式，氢氧化镁就会发生脱水反应，生成氧化镁。将镁合金试件浸泡在高pH溶液中，经过人工腐蚀处理，在试件上形成稳定的氢氧化镁薄膜，然后在673K的空气中加热，进行氧化处理。当镁合金在没有人工腐蚀的情况下在空气中直接加热时，会形成脆性和多孔的氧化膜，这与…的反应相反更多的试件经过人工腐蚀后加热，试件表面涂覆了稳定的氧化膜。在盐浸泡试验中，氧化膜对丝状腐蚀有抑制作用，例如AZ31合金的丝状腐蚀持续时间约为30K，而未处理的样品仅在1.7K后发生丝状腐蚀。氢气泡的演化表明镁的腐蚀反应在盐浸试验的早期阶段被完全抑制。在第二种工艺中，采用电化学方法沉积氢氧化膜，而不是浸泡。氧化处理也是一样，在空气中加热。在3%NaC l溶液中的浸泡试验表明，AZ31合金表面处理后的腐蚀速率为4.2 mg/cm~2/d，而第一种工艺的腐蚀速率为8.6 mg/cm~2/d。使用NaBF4熔盐。将样品在673K的熔盐中浸泡几十K。得到的表面膜由厚度约为10μm的MgF2和NaMgF3组成。采用第三种工艺后，AZ31合金试件在1%NaC l溶液中丝状腐蚀的缓蚀时间延长到1296k(15d)。此外，在1%HCl、1%HNO3等酸性溶液中处理20~30K，仍能保持良好的耐蚀性能。这三种技术都不会降低镁合金的循环利用能力。较少

Three kinds of new technique for surface modification on magnesium and its alloys have been developed. In the first technique, magnesium hydroxide formed in corrosion reaction on magnesium alloys is noticed. The hydroxide is uniformly formed to coat the corroded surface in the early stage of corrosion reactions, and shows a relatively weak corrosion resistance. However, the hydroxide film can not suppress the subsequent corrosion reaction, filiform corrosion. Glancing the chemical formula of the magnesium hydroxide, Mg(OH)_2, leads to the dehydration reaction to form magnesium oxide, MgO. By immersing the specimens of magnesium alloys into high pH solutions, stable films of magnesium hydroxide were formed on the specimens, artificial corrosion treatment, and then the specimens were heated in air at 673K, oxidation treatment. When the magnesium alloys are directly heated in air without artificial corrosion, brittle and porous oxide films are formed, in contrast to the reaction, when the … More specimens are heated after the artificial corrosion, the specimens are coated with stable oxide surface films. The oxide film can suppress the filiform corrosion in salt immersion tests, for example, in the case of AZ31 alloy, filiform corrosion occurred with duration time of about 30ks, while it occurred after only 1.7ks on the non-treated specimen. Evolution of hydrogen bubbles which show the corrosion reaction of magnesium is completely suppressed in the early stage of the salt immersion tests. In the second technique, electrochemical method is adopted to deposit the hydroxide films, instead of immersion. Oxidation treatment is the same, heating in air. Immersion tests using a 3% NaCl solution showed that the corrosion rate of the specimen of AZ31 alloy surface-treated by this technique was 4.2mg/cm^2/d, while that on the specimen treated by the first technique was 8.6mg/cm^2/d. The third technique is the fluoride treatment. Molten salt of NaBF_4 was used. Specimens were immersed into the molten salt at 673K for few tens ks. The resultant surface film was composed of MgF_2 and NaMgF_3 with about 10μm in thickness. The duration time for suppressing the filiform corrosion in 1% NaCl solution was prolonged to 1296ks (15 day) in the case of the specimen of AZ31 alloy by applying the third technique. Moreover, the treated specimens showed corrosion resistance even in the acid solutions such as 1% HCl and 1% HNO_3 solutions for 20 to 30ks. All these three techniques would not deteriorate the recycle ability in magnesium alloys. Less