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

开发了三种镁及其合金表面改性的新技术。在第一种技术中，注意到镁合金在腐蚀反应中形成的氢氧化镁。在腐蚀反应初期，氢氧化物均匀地覆盖在腐蚀表面，表现出较弱的耐蚀性。但是，氢氧化物膜不能抑制随后的腐蚀反应，产生丝状腐蚀。查看氢氧化镁的化学式，Mg(OH)_2导致脱水反应生成氧化镁MgO。将镁合金试样浸泡在高pH溶液中，在试样表面形成稳定的氢氧化镁膜，进行人工腐蚀处理，然后在673K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，而当人工腐蚀后加热更多的试样时，试样表面会覆盖一层稳定的氧化膜。在盐浸试验中，氧化膜可以抑制丝状腐蚀，如AZ31合金，丝状腐蚀发生的持续时间约为30ks，而未经处理的试样仅在1.7ks后就发生了。在盐浸试验初期，表明镁的腐蚀反应被完全抑制的氢泡的演化。在第二种技术中，采用电化学方法沉积氢氧化物膜，而不是浸泡。氧化处理也是一样，在空气中加热。在3% NaCl溶液中浸泡试验表明，该工艺处理的AZ31合金试样的腐蚀速率为4.2mg/cm^2/d，而第一种工艺处理的试样的腐蚀速率为8.6mg/cm^2/d。第三种技术是氟化物处理。采用NaBF_4熔盐。试样在673K的熔盐中浸泡几十k。制备的表面膜由MgF_2和NaMgF_3组成，厚度约为10μm。采用第三种方法，AZ31合金试样在1% NaCl溶液中抑制丝状腐蚀的时间延长至1296ks（15天）。处理后的试样在1% HCl和1% HNO_3等酸性溶液中耐腐蚀20 ~ 30ks。这三种方法都不会降低镁合金的再循环能力。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