Many types of metals, including stainless steel, will naturally develop a protective layer on the surface that limits, hinders or restricts corrosion by exposure to water, air and chemicals in the environment. While this occurs naturally it is slow, so engineers developed processes that allow metals to be passivated quickly and completely.
The Process of Passivation
All passivation, whether natural or engineered, occurs when the chromium molecules on the surface of the alloy react with oxygen to form chromium oxide. This process will continue on the surface allowing stainless to “heal” when the damage is minor and occurs only rarely.
There are natural barriers that can occur on the surface, particularly when stainless is being worked. These barriers to the natural passivation process can include grease or oil, or even scale left behind. With components rubbing, banging, heat, expansion and contraction, as well as scratches or other types of surface damage can expose area of the stainless steel that is no longer passivated, allowing for corrosion. Additionally, chemicals in the environment or other factors may result in uneven natural passivation, so chemical processes can be used to accomplish a uniform and consistent passivated surface layer.
This occurs in two steps and the first is to clean all free iron for the surface. This is done through exposing the stainless to nitric acid or citric acid, both which will dissolve iron. Nitric acid also acts as an oxidizer, helping to speed up the development of the protective passivated surface. Heat and time in the solution will determine the degree of passivation and will be based on several factors, including the specific alloy.
Citric acid is not a natural oxidizer, so it will require exposure to the air to create the protective surface. This is typically a process reserved for smaller parts that are not designed for use in working environments with high heat or chemicals.
Advantages
When choosing passivation as the method for protecting stainless steel, there are several advantages over other types of metal finishing methods. Perhaps the most commonly mentioned is the reasonable cost of the process and the ability to repeat the process as needed on the part or component.
Passivation can be completed as often as needed on parts exposed to extreme types of conditions, including extreme temperature changes, abrasion and wear of parts or even the presence of specific chemicals such as high chloride levels or even the presence of salts or solutions with very low pH levels.
