Jan 2 2002
Topics Covered
Introduction
Contamination by Mild Steel
Contamination by Chloride
Contamination by Carbon
Introduction
Stainless steels with clean surfaces achieve excellent corrosion resistance. Precautions must be taken to prevent contamination of stainless steel surfaces during fabrication, or restoration of surfaces following fabrication process.
Carbon, salt and mild steel are three common contaminants required to be eliminated during manufacture and shipping, as they are capable of affecting the oxide surface layer protecting the stainless steel beneath.
Contamination by Mild Steel
Corrosion due to contamination in mild steels occurs in two ways:
- When mild steel is in contact with stainless steel, there is a chance of breaking a protective oxide film on the surface of steel by forming a corrosion cell, which is a stainless steel-mild steel interface.
- In mild steels, contamination occurs at an outer surface of the oxide layer. Ionic concentration and oxygen depletion underneath the iron deposits will follow corrosion of mild steels under moist conditions. As a result, the oxide film destabilizes and then corrodes.
The following are the major causes of mild steel contamination:
- Contamination due to tools such as screwdrivers, files, drills and polishing tools that have already been used on mild steel
- Contamination from grinding dust produced by using power tools
- Falling particles of welding and flame cutting
- Transportation of unprotected stainless steels in railway wagons
- Using stainless steels to drag mild steels, such as latching stainless steel strips, at the back of truck trays
- Using coil-lifting probes, mild steel hooks, wire ropes, chains and forks for lifting unprotected stainless steel
Contamination by Chloride
Chloride contamination results from the entry of dissolved sodium chloride ions into the protective oxide layer on the metal surface. Following the evaporation of moisture on the metal surface, the chloride ions tend to concentrate on the surface, causing the oxide layer to collapse.
The reasons for the occurrence of chloride contamination include:
- Using hydrochloric acid for chemical etching of concrete floors and cleaning stonework
- Addition of salts to concrete for setting
- Application of salt to melt ice or avert it
- Environmental contamination
- Sweaty hands, seawater and sea spray
Contamination by Carbon
Carbon contamination occurs as a result of a breakdown of organic materials on the surface when a metal is heated, thereby contaminating both molten metal and solid surface.
Carbon contamination occurs due to the following:
- Welding of stainless steel to carbon steel
- Combustion of sooty gas flames, bonding agents, backing strip materials, organic matter, paper and oil films during heat treatment or gas welding
- Paint, pen or pencil markings