Passivation of Stainless Steel – All You need to know
Passivation is a finishing process for stainless steel and other metals. By removing free iron and thus increasing the chromium content in the metal, passivation improves the natural corrosion resistance of stainless steel.
What is passivation?
Passivation is a finishing process for stainless steel and other metal workpieces. It is a non-electrolytic chemical procedure which creates an outer passive layer of shield material. It removes free iron from the surface through the application of nitric acid or citric acid. Thus, the steel parts are less likely to react with air and therefore show better corrosion resistance.
As a consequence of removing free iron and therefore increasing the chromium content in the metal, the passivation process achieves a thicker chromium oxide layer. This results in a significantly improved corrosion resistance of steel and other metals.
Why should stainless steel grades get passivated?
Stainless steel is naturally great corrosion resistant. However, it is not completely immune to corrosion, especially rust. Stainless steel owes the great corrosion resistance to the high chromium content in the alloy. Due to the chemical reaction of chromium and oxygen, stainless steel forms a protective thin layer of chromium oxide on the surface. However, in spite of the protective layer, small spots on the surface may begin to rust. This happens because water molecules, due to grain boundaries or foreign matter, are able to oxidize some iron in the spots. This process is called rouging.
The removal of the contaminants and the oxide growth process is called passivation.
How does passivation work?
There are several specifications for the passivation process of steel and other metals, depending on the different standards (ASTM A967, AMS2700, ASTM A380). Mostly, the different standards have some steps in common. First, the metal surface gets cleaned from surface contamination and particles of iron. Second, the workpiece gets chemical treatment via immersion. This step takes place in an acid bath, usually in a nitric acid solution or citric acid. And finally, the stainless steel surface gets tested whether the chemical treatment was successful.
The procedure accelerates the natural process which occurs when the material is exposed to oxygen in the atmosphere. Thus, it helps to build the inert, passive layer in a quicker way. Further, the build-up protective oxide layer becomes significantly thicker than found in nature.
The process in detail
As already mentioned, all different passivation specifications have the following steps in common. The first step in the procedure is cleaning of the workpiece. The steel must be perfectly clean of any contaminants. This circumstance is proven by a validating test. After the cleaning, the parts are placed in an acidic passivating bath. Depending on the stainless steel grade, one of these approaches can be used — nitric acid passivation, nitric acid with sodium dichromate passivation and citric acid passivation. The duration, which usually has a minimum of 20 to 30 minutes, and the temperature, which ranges from ambient to 60° C, of the process is predetermined. Next, the parts are neutralized through a bath of aqueous sodium hydroxide. Finally, the steel workpiece gets rinsed with clean water and dried. To validate the results of the process, the surface undergoes a testing procedure with humidity, elevated temperature or with salt spray.
Martensitic stainless steel, however, must receive a special treatment due to it is difficult to passivate.
How to further improve wear resistance
In certain application areas, the natural passivation may not suffice to protect the material from corrosion and wear. In these cases, BorTec provides the perfect solution. The EKABOR® technology increases the wear protection of unalloyed and low alloyed steels through boriding. Further, Borocoat® diffusion layers increase the layer hardness and improve the resistance of adhesive and abrasive wear of steel. For further information about our products, don’t hesitate to contact us!