Surface contaminants, defects, and contamination arising through fabrication are all potentially detrimental to the oxide film that protects stainless steel. Corrosion can be initiated from the moment stainless steel has been damaged. Therefore, it is imperative that your factory remains free of potential contaminants.
Carbon Steel Contamination
One of the most common contaminants that can affect stainless steels during fabrication is carbon steel. During the fabrication process, it is critical that prevention methods are in place to stop dust and grime from settling on stainless steel surfaces.
Due to this, it is common practice to keep stainless steel fabrication and welding activities segregated from any carbon steel fabrication work. When working with carbon steel, there is undoubtedly going to be grinding dust and welding spatter. This will settle onto the stainless steel, the iron particles will rust rapidly, inevitably leading to corrosion.
How to avoid contamination
It can be as simple as walking on iron particles and transferring, then embedding them into the surface of the stainless steel. During the layout and handling of stainless steels, extra care must be taken to protect them from any chance of contamination.
Protective coating films should remain on stainless steels until it is essential to remove them for fabrication or installation purposes. This minimises the risk of damage to the stainless steel structure.
Surface damage on stainless steels such as scratches and gouges can allow entrapment of contaminants, providing ideal locations for corrosion. Depending on the object that caused the surface damage, the scratches or gouges may contain carbon steel or other contaminants. These areas should be mechanically cleaned to prevent corrosion when this occurs.
Other products can contain contaminants that lead to localised corrosion or act as a barrier against chemical and electrochemical cleaning. These include:
- chalk marks
These sorts of products should be removed or not used around stainless steel manufacturing.
Preventative measures in the welding process
During the welding of stainless steels, care must be taken to control interpass temperatures and heat input. Austenitic stainless steels are typically limited to temperatures ≤150°C with a max heat input of 1.5Kj/mm.
When these guidelines are exceeded carbon can precipitate in the grain boundaries (underneath the metal surface) of stainless steels at high temperatures. The carbon combines with chromium to form chromium carbides. Chromium is the element that gives stainless steel its corrosive properties. This depletes the area beside the grain boundaries of chromium, leading to corrosion.
Use of rotary wire brushes, grinding discs and hand tools
It is common practice (and sometimes essential) in metal fabrication workshops to use rotary wire brushes. They can be used for an assortment of tasks including but not limited to:
- rust and oxide removal
- surface preparation
- weld cleaning
- surface finishing
Rotary brushes are versatile and are not as easily blocked with particles and debris during the process of removing rust or coatings. They can be used to play an active role in reducing the risk of surface contamination. However, if a wire brush has been used on carbon steel, it cannot be used on stainless steel as contamination can occur.
When brushes come into contact with carbon steel particles, they can become embedded into the surface of the steel. This leads to surface corrosion. For this reason, stainless steel brushes should be stored away from any areas where they may come into contact with carbon steel particles. This includes common areas such as workbenches.
All grinding discs should be treated the same as above. The grinding particles that clog up the grinding discs are easily transferred to the stainless steel.
It is good practice to label or even colour code tools to ensure operators never use the wrong ones on the wrong materials.
Covering of pipe stands, tressels and vices
Often during fabrication, it is essential to use pipe stands or vices to ensure components are level and supported. A common mistake when fabricating stainless pipework is not covering the vice or pipe stand.
Vices can contain iron particles in the jaws. When the jaws are clamped onto stainless material the iron particles become embedded into the surface, leading to corrosion. The use of clean rags and/or tape can mitigate this risk
Pipe stands can also transfer iron particles to the surface of stainless steel. They should be covered using clean, fresh rags or tape.