Overview

What are the two most powerful corrosive agents in a process plant? Not acids or sulfur, or caustic or salt. Neither cyanides nor carbonates nor chlorides. The most aggressive corrosive agents are air and water. Nitrogen is inert. It is oxygen in an aqueous environment that is the main cause of many corrosion problems.

Rusty carbon-steel pipe is the commonly encountered form of corrosion due to air and water. Stainless steel appears to be immune to such corrosive attack, but this is not quite true. Stainless steel contains a substantial concentration of chrome and nickel. Common 304 stainless steel has 18 percent chrome and 8 percent nickel.

The chrome especially is subject to oxidation when exposed to wet air. But the metallic oxides so formed adhere tightly to the surface of the metal. This rugged metallic oxide layer acts as a barrier to prevent further destructive oxidation of the base metal. The oxide layer is so thin that it is smaller than the wave length of visible light. Thus, it is quite invisible to the human eye. Stainless steel piping appears shiny after years of service, even though its surface has been oxidized after just a few minutes of service.

Anyone who has ever painted a wrought iron fence to prevent rust appreciates the concept of a protective barrier. Aluminum and titanium are protected from corrosion due to the rapid formation of a surface metallic oxide layer, which tightly adheres to the metal surface.

The problem with ordinary carbon...