Introduction
"My stainless steel has gone rusty. It can't be stainless. What's gone wrong? Who's to blame? How can I fix it?"
Although the vast majority of stainless steel applications work perfectly as intended by the designer and fabricator, there are a significant number of instances where someone, often the end-user, is disappointed by the performance of the material. The causes of these disappointments tend to fall into only a few basic categories. In nearly all cases, a little basic knowledge would have prevented or significantly improved the situation.
On the assumption that prevention is better than cure, this short article addresses these issues.
Main Issues
The causes of disappointment can arise at any point in the long supply chain that often applies to a stainless steel project. This helps to explain why problems occur. Getting the appropriate knowledge to all parts of the supply chain is difficult and it only takes ignorance in one small part to create a problem later on. The main issues are:
1. Importance of surface finish in determining corrosion resistance
Lack of knowledge in this area is a major cause of problems. Most specifiers and designers understand the importance of selecting a grade of stainless steel, for example 1.4301 (304) or 1.4401 (316). But surface finish is at least as important. The subject is fully explored in The Importance of Surface Finish in the Design of Stainless Steel.
Briefly, a bright polished surface gives maximum corrosion resistance.
A directional polish equivalent to the EN 10088-2 2K (Ra = 0.5 micron max), usually produced using silicon carbide (SiC) abrasives, will give adequate corrosion resistance in many severe environments notably heavy urban and coastal ones.
A common surface finish achieved with 240 grit alumina abrasives has been implicated in the corrosion of stainless steel in urban and coastal environments. In some cases, surface roughness Ra values have been measured at well above 1 micron which is known to be inadequate in these environments.
The lack of any specified surface finish on architectural drawings can be the source of the final problem.
If, at any stage of the supply chain, there is any doubt about the appropriate surface finish, specialist advice should be sought.
2. Importance of post-fabrication treatments
Apart from some specialised processes, welds in stainless steel always result in some degree of heat tint. Heat tint is essentially an oxidised surface which has a reduced corrosion resistance compared to the parent material. Therefore, the normal practice is to carry out some form of post weld treatment to improve the corrosion resistance.
Details of these procedures can be found at:
Post Weld Cleaning and Finishing of Stainless Steels
Welding and Post Fabrication Cleaning for Construction and Architectural Applications
Good fabrication practice always includes post weld treatment. Failure to do so can give rise to unnecessary cost of rectification later on.
3. Importance of segregating carbon and stainless steel
Sometimes "rusting" of stainless steel turns out to be nothing of the kind. It is the rusting of carbon steel which has contaminated the surface of the stainless steel at some point in the production process. Possible sources of contamination from carbon steel include:
Tools
Lifting Gear, Ropes, Chains
Grinding dust
Cutting sparks
Wire brushes
Wherever possible, stainless steel and carbon steel should be fabricated in separate areas of the workshop or better still in separate workshops. Where not possible it is important to clean down machines used for carbon steel before using them for stainless steel. Stainless steel surfaces should be protected with plastic coatings for as long as possible.
4. Importance of site management
It is quite possible for everything to be done well in fabrication, only for the whole project to be spoiled by inappropriate practices on site. The issues outlined in 3. apply just as much to the site installation as anywhere else in the process.
In addition, it must be remembered that what is appropriate for one building material is totally unacceptable for another. For stainless steel it has to remembered that masonry and brick cleaners may contain hydrochloric acid sometimes called muriatic acid. If these fluids are to be used at all near stainless steel, care should be taken to protect the stainless steel surfaces. If splashes occur, they should be immediately washed off with water. Failure to do so will result in serious attack of the stainless steel resulting in expensive rectification costs
5. Importance of choosing correct grade for the application
This aspect almost goes without saying. It is only this far down in the list because it usually is considered. But if the "wrong" grade has been chosen the consequences can be severe. Some guidelines on material selection are given here.
6. Cleaning and Maintenance
Some people think that stainless steel's corrosion resistant surface somehow repels dirt and other contaminants. Like any surface stainless steel requires some maintenance. Guidance on this aspect can be found at Cleaning Methods for Stainless Steel
7. Importance of seeking technical advice in cases of doubt
If there is any doubt about the correct choice of grade, surface finish or other aspect of a prospect involving stainless steel, the following advisory services can be consulted:
Stainless Steel Advisory Service
If these simple guidelines are followed the number of disappointed users will be reduced
Source: http://www.bssa.org.uk/
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