Michel Basson, manager of sassda’s Western Cape branch, talks about the vital role of the chrome-rich passive layer on stainless steel and how inadvertently blocking the layer’s access to oxygen can ruin its corrosion resistance.
Stainless steels are inherently corrosion resistant materials that do not need additional surface protection such as painting, galvanising or even anodising to enhance their appearance and lifespan.
The reason for this is that stainless steels are protected by a thin surface layer that is the result of chromium (in the metal) reacting with environmental oxygen. The strength of the chromium-rich oxide layer and thus the corrosion protection is based on the level of chrome in the material and the availability of oxygen in the surrounding atmosphere.
Unlike rust (also a surface layer), the surface layer on stainless steel does not readily react with common substances, hence the phrase, passive layer. Like human skin, this layer has the ability to repair itself provided that the affected surface is clean and has access to environmental oxygen. This explains the fact that only minor routine and simple maintenance and cleaning is needed to keep stainless steel surfaces in good condition so that the aesthetic appearance and corrosion resistance are not compromised.
Techniques such as the SURFOX electrochemical weld cleaning system are highly effective in removing heat tint in the heat-affected zone of welded stainless steel.
During fabrication this layer is often damaged or even destroyed in areas where cutting, grinding and welding take place. Forming the metal can also cause damage, but most fabricators protect against this by applying a PVC/PE film to the material surface, which is only removed once fabrication is complete. It was mentioned that this layer has the ability to form and maintain itself naturally in an oxygen-containing environment. It is also true that the environment can also contain some corrosion agents that might be able to attack the metal whilst the passive layer is still underdeveloped and possibly weak.
This is the reason why fabricators not only clean (mechanically or through chemical pickling) potentially damaged areas properly, but they also chemically passivate these areas to ensure full protection as soon as the fabrication process and subsequent cleaning is completed.
The case study below illustrates the importance of ensuring passivation has been completed before any stainless steel products are exposed to their corrosive environments.
A company in Cape Town designs and supplies fish processing equipment to factory ships working from the Cape Town harbour. The company used mainly Grade 304 material since regular cleaning takes place with clean water once the ship is operational, which reduces the risk of pitting corrosion – and in the past, this has worked well.
In order to be competitive, the company sources out some of its frames and other non-critical components to other manufacturers. Whilst the company head office and workshops are located a stone’s throw away from the ocean in Table Bay, the supplier of the square tubing frames is based at a facility in the Boland, about 65 km inland. This contractor supplied 304 frames with a brushed finish to the fish processing equipment manufacturer on a Friday afternoon. The frame was wrapped in transparent plastic, but was partly uncovered for a quality inspection on delivery. The frame was accepted and everyone left for the weekend.
The technical staff were shocked to discover on Monday morning that the frame looked like it had measles, with brownish blemishes all over the exposed surfaces and small dark spots that suggested the early stages of a pitting corrosion attack. Sassda was called to investigate. Luckily both companies function in an ISO 9000 environment and tracking all the manufacturing steps was fairly simple.
The following came to light.
The material could be traced to its original material certificate and all was found to be in order. The next step was to retrace the manufacturing process. This was done and all relevant staff were interviewed. Due to some production problems, the manufacturing of the frame fell behind schedule. The delivery on the specific Friday was non-negotiable and in order to save time, the finishing department started to cling wrap the frame as the polishing crew went about blending the welds to the frame‘s brushed finish. This was evident in hindsight since the packaging material did show some signs of heat damage – it became slightly melted and shrunk where the frame was still hot from polishing.
Once these dual processes were completed, the frame was loaded and delivered to the customer in Cape Town.
Analysis of all known facts at this stage was able to indicate the reason for discolouration of the frame.
During the polishing process, the protective passive layer is removed, exposing the material below the layer to the environment. These areas need access to oxygen, through contact with the environment or chemical treatment, to restore the passive layer.
Access to oxygen was not possible, however, since the frame was immediately wrapped in ‘airtight’ cling wrap. To allow for a delivery inspection, upon arrival at the customer’s premises close to the ocean, the unprotected surfaces were then exposed to the ocean environment, which contains high levels of chlorides, along with pollution from the surrounding industrial area. This allowed the marine environment to enter the plastic packaged frame. Over the weekend, temperatures rose to over 30 ˚C in the storage area where the frame was kept and the humidity was also high. Thus, on Monday morning, the frame was ready for use as a sample at a corrosion lecture. The surface layer was destroyed and the damage was evident within a weekend.
Luckily, the corrosion was superficial and could easily be removed. Cleaning the frame with soap and abrasive pads quickly solved the problem. It was then treated chemically to ensure the full development of the passive layer. The frame was then installed, on time, onto the factory ship.
As far as we know, it is still on the ship and fully functional.
This illustrates just how quickly things can go wrong if stainless steel is not properly passivated. Stainless steel is only stainless if the chromium- rich passive film is allowed to form a complete protective layer over the whole surface of the material. To do so, it needs regular access to oxygen, particularly immediately after any fabrication processes involving welding, grinding or polishing and, while in service, immediately following any potential or real damage that has been caused to the surface.
In principle, the corrosion resistance of stainless steel depends on the surface oxide film being chromium rich. The higher the chromium content of this oxide layer, the better the corrosion resistance.