镁合金稀土转化膜研究进展_李凌杰

镁合金稀土转化膜研究进展

Progress in Research of Rare Earth Conversion

Coatings on M agnesium A lloy s

李凌杰1,雷惊雷1,张胜涛1,李荻2,田钰靖1

(1重庆大学化学化工学院,重庆400044;

2北京航空航天大学材料科学与工程学院,北京100083)

LI Ling-jie1,LEI Jing-lei1,ZH AN G Sheng-tao1,LI Di2,

T IAN Yu-jing1(1College o f Chemistry and Chemical Eng ineer ing, Cho ng qing U niversity,Chongqing400044,China;2Schoo l o f Mater ials Science and Engineering,Beijing U niversity of Aero nautics and Astro nautics,Beijing100083,China)

摘要:镁合金稀土转化膜技术是近年来发展起来的一种环保型镁合金表面处理新技术。本工作从成膜工艺及耐蚀性能评价,膜的组成、结构及形貌,膜的形成及其机制,膜的耐蚀机理研究四个方面综述了国内外在镁合金稀土转化膜研究中的进展及现状,指出了目前镁合金稀土化学转化处理中存在的一些问题,并展望该技术的发展前景。

关键词:镁合金;稀土转化膜;表面处理

中图分类号:T G178;T G146.2+2文献标识码:A文章编号:1001-4381(2006)10-0060-05

Abstract:Rare earth conver sion coatings have been developing as an environmenta-l friendly surface tr eatment technolo gy fo r magnesium alloys.The pro gress and current situatio n in researches of this technolog y w ere r ev iew ed.T he pr oblem s that this technolog y faced at present w ere also pointed out as w ell as the prospects in the future.

Key words:magnesium alloy s;rare earth conversio n coating s;surface technolog y

镁及其合金具有密度小、比强度高、加工焊接和阻尼性能好以及尺寸稳定、价格低廉、可回收利用等优点,在机械制造、汽车制造、航空航天、电子、军事、通讯、光学仪器和计算机等领域具有广阔的应用前景。但镁及其合金的耐蚀性能很差,在潮湿大气或酸性、中性、弱碱性溶液中都容易发生腐蚀,这在很大程度上阻碍了镁合金发挥其性能优势。因此,进行适当的表面处理以增强镁及其合金的耐蚀性能对于拓展和加快镁合金的应用具有重要意义。

化学转化膜技术是镁合金表面防腐蚀处理的有效方法[1]。镁合金基体与转化处理溶液相接触,在基体表面形成一层转化膜,这层膜不仅本身具有较好的耐蚀性能可保护基体免受腐蚀介质的侵蚀,还可作为涂装等后续处理的底层,并且该方法处理工艺简单易控,因此应用较为广泛。但由于镁合金化学转化处理溶液中通常含有毒性高且致癌的六价铬,随着人们环保意识的增强,六价铬的使用正受到严格限制,因此,开发新型、高效、环保的镁合金化学转化处理技术非常迫切和重要。

稀土化学转化处理由于其无毒、无污染的特点,并且对铝、锌等金属及其合金[2-15]以及碳钢[16,17]和不锈钢[18-22]等多种金属材料所表现出的优良的防护效果,已经引起从事镁合金表面处理的研究者们的重视。自从2000年Rudd等[23]首先报道了对纯镁和WE43镁合金进行稀土盐化学转化处理的结果,近年来人们陆续开展了镁合金稀土转化膜方面的研究。本工作从成膜工艺及耐蚀性能评价,膜的组成、结构及形貌,膜的形成及其机制,膜的耐蚀机理研究几个方面对国内外在镁合金稀土转化膜研究中的进展和现状进行了较为详细的评述。

1成膜工艺及耐蚀性能评价

1.1化学浸泡工艺

化学浸泡法是将镁材料置于含稀土盐的溶液中浸泡一段时间使材料表面形成稀土转化膜的方法,因其操作简便此工艺在镁及其合金的稀土化学转化处理中应用很多。根据成膜溶液的组成,该工艺可以分为两类:一类为单一稀土盐溶液浸泡;另一类则为含有添加剂的稀土盐溶液浸泡。

单一稀土盐溶液浸泡处理工艺及处理后试样的耐

蚀性能参见表1。这类处理液多为稀土的硝酸盐溶

液,其中使用最多的是硝酸亚铈溶液。可以看出,经浸

表1 单一稀土盐溶液浸泡处理工艺及试样的耐蚀性能评价结果T able 1 T echnolog ical parameter s of sole rare ear th salt solution immersion

tr eatments and co rresponding anticor ro sive perfor mance

Substrate material

Treatm ent condition

Anticorrosive perform an ce

Process numb er/reference source

M g(99.9%),WE43

Placed th e sub strates in solu tion s of

50m mol/L C e(NO 3)3,La(NO 3)3,or Pr (NO 3)3at r oom tem perature un der gentle agitation for 5mins.

Th e coatings significantly redu ced the disso -lution of magnes ium in th e pH 8.5buffer solution.W ith continued im mer sion in the aggressive solution,th e coatings first b e -cam e more protective,b ut began to deterio -rate after periods exceeding 60m ins.

1/[23]

M g(99.98%),AZ91D,AM 60B

Imm ers ed in solutions of 3g/L Ce

(NO 3)3,CeCl 3,

Ce (SO 4)2,

L a

(NO 3)3,or Nd (NO 3)3at 40b C for

20m ins.

Rare earth conversion treatm ent improved th e corrosion resistance of the materials ex -cept AZ91D to some extent.

2/[24]AZ31B

Placed in 50mm ol/L Ce(NO 3)3solution

(pH =512)at 30b C under agitation for 20m ins.

Unreported

3/[25]

泡处理后试样的耐蚀性能普遍提高。

表2列出了含有添加剂的稀土盐溶液浸泡处理工艺及处理后试样的耐蚀性能。添加剂既可以包括用于缩短成膜时间的H 2O 2等强氧化剂,如工艺5,6;也可以包括其他参与成膜的物质,如工艺4处理液中的

Mg (OH )2以及铝的化合物AlF 3,Al 2O 3#x H 2O,它们对改善膜层性能有一定的作用。

总体上,这类工艺得到的膜与基体的结合力较差,耐蚀性能不太理想。为了进一步提高膜层的防护效果,需要对试样进行其它处理。

表2 含有添加剂的稀土盐溶液浸泡处理工艺及处理后试样的耐蚀性能评价结果T able 2 T echnolog ical parameter s of immer sion treatments with so lutio ns containing

r are earth salt and additives,and co rr esponding anticor ro sive perfor mance

Substrate material

Treatm ent condition

Anticorrosive perform an ce

Process numb er/reference source

M g(99.98%),

AZ91D

Carried out in th e M g (OH )2s aturated

Ce (NO 3)3@6H 2O aerated solution w ith addition of AlF 3an d Al 2O 3@x H 2O at ambient tem perature for dif -ferent immersion periods.

Th e tr eatm ent provided a shift of corrosion potential tow ards m ore positive values.T he addition of alumin um compoun ds in the tr eatm ent s olution further improved the ben eficial effects of the cerium treatmen t.4/[26]

AZ63

Imm ers ed in 10mg/L CeCl 3aqueous so -lu tion added w ith differen t amounts of

33%H 2O 2(up to 50mL/L)at room temperature for different immersion

tim e (up to 180s )under gentle agita -tion.

Th e treatment im proved the pitting poten -tial of the alloy.Better corrosion res istant surfaces w ere obtained w hen the samples were imm ers ed repeatedly in th e conversion bath for 30s up to a total immersion tim e of 180s.The excess of hydrogen peroxide con -centration and too man y repeated imm er -s ions both redu ced the corrosion res istan ce of the conversion coatin g.

5/[27]M g(99.8%),AZ91,AM 50

Immersed in aerated 0.05mol/L CeCl 3aqueous solution added w ith 25mL/L of

33%H 2O 2at room temperature by re -peated 30s immersion follow ed by rinsing in deionized water and drying w i th air up

Th e coating imp roved th e corrosion resist -ance of pure magnes ium and magnesium al -loy s in chloride media.

6/[28]