Stainless Steel Corrosion: The Question of Rust

Stainless steel is known to be extremely resistant to corrosion, a quality that makes it far superior to almost any other metal. It is, however, important to note that 'stainless' does not mean 'rustless'! The material can be vulnerable to rusting due to a number of factors, some of which we will look at in more detail in this edition of the BS Stainless blog. 

What makes stainless steel so strongly resistant to the damaging effects of corrosion is its passive layer of chromium oxide, which forms spontaneously on its surface. When this layer is damaged in some way, it is unable to protect the underlying metal which then becomes susceptible to rusting. 

Crevice Corrosion
Tiny gaps in stainless steel constructions, such as imperfect joints, can provide a place for fluid to accumulate. Over time, the amount of oxygen in the fluid depletes; when the surface of stainless steel is deprived of oxygen, it inhibits the ability of the passive layer to reform. Simultaneously, the amount of chlorides in the fluid build, attacking the stainless steel. The result is referred to as crevice corrosion. 

Galvanic Corrosion
When two different kinds of metal are in contact with one another in the presence of a conductive electrolyte (such as water), galvanic corrosion can occur. Using the electrolyte as a medium, electrons flow from one metal (the anode) to the other (the cathode). If the stainless steel is acting as the anode, this loss of electrons can result in corrosion. 

Contamination
If small pieces of particulate matter become embedded in the stainless steel's surface, this can cause corrosion. It is in fact the contaminating matter (such as small particles of carbon steel or iron), that is rusting and not the stainless steel itself. Various chemicals such as chlorine and the salts in seawater can also act as contaminants, damaging the passive layer and resulting in corrosion.

Improper Maintenance
As noted in the previous edition of our blog, keeping stainless steel clean is essential to ensure that it continues to perform optimally. It is also important to use the right cleaning chemicals as strong alkalies and acids can do more harm than good. In addition, stainless steel can be damaged by using abrasive cleaners which create scratches that breach the material's passive layer. 

There are many ways to prevent rust from occurring in stainless steel, including choosing the right grade and finish. This is where BS Stainless can help. Not only do we supply a comprehensive range of stainless steel materials in all available grades and finishes, our skilled technical team can offer authoritative advice on choosing exactly the right combination to suit any given application. Find out more on our website.