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2．不锈钢酸洗钝化原理
不锈钢的抗腐蚀性能主要是由于表面覆盖着一层极薄的(约1nm)致密的钝化膜，这层膜隔离了腐蚀介质，是不锈钢防护的基本屏障。不锈钢钝化具有动态特征，不应看作腐蚀完全停止，而是形成扩散的阻挡层，使阳极反应速度大大降低。通常在有还原剂(如氯离子)情况下倾向于破坏膜，而在氧化剂(如空气)存在时能保持或修复膜。 不锈钢工件放置于空气中会形成氧化膜，但这种膜的保护性不够完善。通常先要进行彻底清洗，包括碱洗与酸洗，再用氧化剂钝化，才能保证钝化膜的完整性与稳定性。酸洗的目的之一是为钝化处理创造有利条件，保证形成优质的钝化膜。因为通过酸洗使不锈钢表面平均有10μm厚的一层表面被腐蚀掉，酸液的化学活性使得缺陷部位的溶解率比表面其它部位高，因此酸洗可使整个表面趋于均匀平衡，一些原来容易造成腐蚀的隐患被清除掉了。但更重要的是，通过酸洗钝化，使铁与铁的氧化物比铬与铬的氧化物优先溶解，去掉了贫铬层，造成铬在不锈钢表面富集，这种富铬钝化膜的电位可达+1．0V(SCE)，接近贵金属的电位，提高了抗腐蚀的稳定性。不同的钝化处理也会影响膜的成分与结构，从而影响防锈性，如通过电化学改性处理，可使钝化膜具有多层结构，在阻挡层形成CrO3或Cr2O3，或形成玻璃态的氧化膜，使不锈钢能发挥最大的耐蚀性。
国内外学者对不锈钢钝化膜的生成进行了大量研究。以近几年北京科大对316L钢钝化膜光电子能谱 (xps)研究为例作简述。不锈钢钝化是表面层由于某种原因溶解与水分子的吸附，在氧化剂的催化作用下，形成氧化物与氢氧化物，并与组成不锈钢的Cr、Ni、Mo元素发生转换反应，最终形成稳定的成相膜，阻止了膜的破坏与腐蚀的发生。其反应历程为： Fe·H2O + O*≈[FeOH·O*]ad + H+ + e [FeOH·O*]ad≈[FeO·O*]ad + H+ + e [FeO·O*]ad + H2O≈FeOOH + O* + H+ + e [FeO·O*]ad≈FeO+O* FeOOH + Cr + H2O≈CrOOH + Fe·H2O 2FeOOH≈Fe2O3 + H2O
2CrOOH≈Cr2O3 + H2O MO + 3FeO + 3H2O≈MOO3 + 3Fe·H2O Ni + FeO + 2H2O≈NiO + Fe·H2O
其中O*表示钝化过程中的催化剂，且在钝化过程中浓度不变，ad表示吸附中间体。可见，316L钝化膜最表层存在Fe2O3、Fe(OH)3、或γ -FeOOH、Cr203、CrOOH或Cr(OH)3、MO以MOO形式存在，钝化膜主要成分为CrO3、FeO与NiO。
The Principle of the stainless steel pickling and passivation
The main reason of the corrosion resistance for stainless steel is that there is a very thin (about 1nm) compact passivation film on the surface of stainless steel. This 1nm passivation film is the basic protection for the stainless steel. Stainless steel passivation has dynamic feature. It means that corrosion is not ceased and dispersive barrier film is formed on the surface of the stainless steel. The film reduces the speed of anodic reaction. Generally, the film will be damaged by reductant, the film will be maintained or repaired by oxydant.
When we put the stainless steel workpiece in the air, oxide film will be formed on the surface of it, but this film’s protective value is not perfect. Generally, procedures are as following: first, cleaning the workpiece throughly (including alkaline cleaning and acid cleaning), then use the oxydant to do the passivation to ensure the passivation’s completeness and stability. One of the purposes of acid cleaning is to create a good condition for passivation treatment and a good passivation film can be formed. Through the acid cleaning, the surface(average thickness 10μm) of the stainless steel will be corrosive. The acid’s chemical activity makes the dissolution rate in defect area higher than other area, so the whole surface of the stainless steel will be average thickness, the original hidden trouble that can cause the corrosion easily will be eliminated. But the most important thing is that, iron and iron oxides will be dissolved preferentially than
chromium and chromium oxides through the pickling and passivation, resulting in the enrichment of the chromium on the surface of the stainless steel. The potential of the chromium-rich passivation film will reach +1.0V(SCE). Because the potential is close to the potential of the precious metal, the stability of the corrosion resistance is improved. Different passivation treatment also will affect the composition and structure of the film and the ability of the corrosion resistance. In order to reach the maximum ability of the corrosion resistance of the stainless steel, we can make the passivation film has multilayer structure through the electrochemical treatment, and in the barrier layer, CrO3 or Cr2O3 is formed, or glass state oxide film is formed. Domestic and foreign scholars have done lots of researches on the formation of the passivation film of the stainless steel. For example, in recent years, the scholars of the University of Science and Technology Beijing have studied the X-ray photoelectron spectroscopy of the passivation film of the 316L steel。The passivation of the stainless steel is that, for some reason, the surface layer of the stainless steel dissolved and adsorbed with water molecule, under the catalysis of the oxidizing agent, oxide and hydroxide will be formed. Conversion reaction will take place between oxide/hydroxide and Cr, Ni, Mo which are the elements of stainless steel. Ultimately, the steady film will be formed and it will prevent the film from damage and corrosion. The reaction processes are as following:
Fe·H2O + O*≈[FeOH·O*]ad + H+ + e [FeOH·O*]ad≈[FeO·O*]ad + H+ + e [FeO·O*]ad + H2O≈FeOOH + O* + H+ + e [FeO·O*]ad≈FeO+O* FeOOH + Cr + H2O≈CrOOH + Fe·H2O 2FeOOH≈Fe2O3 + H2O
2CrOOH≈Cr2O3 + H2O MO + 3FeO + 3H2O≈MOO3 + 3Fe·H2O Ni + FeO + 2H2O≈NiO + Fe·H2O
O* is the catalyst, and during the passivation, its concentration will not change. ad is adsorbing material. It can be seen that there are Fe2O3, Fe(OH)3 or Y-FeOOH, Cr2O3, CrOOH or Cr(OH)3 on the top layer of the passivation film of the 316L steel. MO exists in the form of MOO. CrO3, FeO and NiO are the main constituents of the passivation film.