Sassda was recently called upon to comment on the visible corrosion and pit marking on a stainless steel railing system as is installed in a biokinetics pool. The handrailing used for assisting in stabilising and support users once in the water for treatment. Within 6 month of the pool being in use after the installation, the first signs of visible corrosion and even pitting became apparent. The pool was filled with a saltwater solution within chloride and chlorine levels range of grade 316L material in terms of adequate corrosion protection. The pool temperature is maintained between 32 – 34 degrees Celsius. The pool is in an enclosed environment, but with a high roof allowing humidity levels to stay low. The grade of stainless steel used was confirmed by spectrometer testing to be the specified austenitic grade 316L.
As seen from the picture on the right, the components are submerged in the saltwater solution, it is never cleaned. However, the components are exposed to water being constantly circulated and therefore not oxygen deficient or stagnant. The interesting part to this investigation is the fact that the grade 3126 railing system was installed to replace a grade 304 system that was in use for 4 years before the first signs of discoloration and possible corrosion. The redundant equipment was still available and spectrometer test confirmed the old set to be grade 304 stainless steel.
The picture on the left shows the damage to the material surface caused by pitting corrosion. At the lower right, the very rough surface finish is visible.
On the right, the mounting to the pool structure indicates crevice corrosion activity. Note the unsealed crevices, lack of post-weld treatment and incorrect grain direction in the tube surface finish.
Failure of components are not usually caused by one incident or reason only. It is common for failure to be attributed to a series of smaller and less critical factors. It is therefore important to discuss the following aspects.
Material grade
It was mentioned that the previous installation was manufactured from grade 304 stainless steel and the new installation was manufactured in grade 316. The owners of the pool report a good lifespan and performance from the original 304 stainless steel, but that the 316 grade installation showed signs of pitting as early as 6 months since starting operation. This should not be the case since grade 316 contains molybdenum making it especially protected in high chloride environments such as this. It is expected that 316 would perform better in this type of application. It is noteworthy that technically the original grade 304 would not be suitable for the application since it lacks the specific protection against pitting corrosion offered by the molybdenum content in grade 316. However, the installation shows the opposite with the 316 underperforming.
Material surface finish
It would normally be required in this application to make use of very highly polished surfaces. Sassda normally would not advice using any surface finish less than #600 grid in this scenario. Judging from the visual evidence, the finish is not up to requirement and it can be as rough as the equivalent of #180 grid. It is also noticed that the finish was done with a circumferential grain direction. This means that when the components are installed horizontally, the grain direction would point vertically, and water (and possible contaminants) can drain off. However, if the component is installed vertically the grain direction becomes horizontal and the small ridges impede the washing off to remove any contaminants or chloride deposits in this case. The visual evidence also points to the fact that welding was not properly smoothed down as expected for this type of application. Explained in very simplistic terms: the equipment should have had at least the same shiny surface smoothness as the handrailing and similar structures that we find at airports and public places.
Exposure to salt or harmful chemicals
It was already commented on the fact that the choice of grade 316 stainless steel was correct. It was also established that the surface finish was not to the required standard. This would potentially limit the performance of the stainless steel and the evidence points to this fact. Even with the occasional spike in chloride and chlorine levels in the pool water, it would not be expected that 316 would deteriorate at this rate. The risk of pitting is also addressed by the fact that the water circulating in the pool will prevent stagnant conditions. Thus, there will be adequate oxygen levels in the water to maintain the protective chrome-oxide layer on the material surface. During the investigation no other chemical or forms of pollution could be identified as a contributing factor.
Cleaning frequency and method
It was mentioned in the initial observations that the equipment remains submerged and would therefore not have a standard cleaning regime. Any chloride build up will be washed away during use and the circulating water. No additional cleaning regime is in place or chemicals are used. Maintenance, or the lack thereof, cannot be a contributing factor in this case.
Fabrication
Several fabrications and design issues came to the fore. The following issues stand out as part of the investigation:
• Inadequate surface finish as explained earlier. This should not be a real fabrication issue since the tube can be bought from distributors in the required finish. This might be a case of ignorance regarding forms and finishes available.
• The weakest points in any assembly to be used in corrosive condition are always the joints, whether bolted, welded or otherwise. In the joint areas (as visible in Figure 3) the risk for crevice corrosion is elevated when joints and seams are not completely sealed off. The water in the crevice will be stagnant and the passive layer in this localised area will be impeded. Further to this, a secondary galvanic effect will happen in the crevice when hydrogen breaks away from the water molecule, binding with chloride ions to form hydrochloric acid in the crevice. That is why the corrosive byproducts are so abundant at the joining points at the floor and wall. Welded joints are also risk points since the metallurgy of the weld will differ from the parent material. Welds should preferably be purged welds to ensure maximum weld integrity.
• As mentioned, welding on this product seems to be done with an electrode. This gives rise to excessive heat in the welded area which will also impact negatively on the weld metallurgy as described earlier. Welds for this type of application should be much more elegant to control heat input and to protect the metallurgy of the joint.
• Some of the components have been exposed to extreme forms of corrosive attack over large continuous areas. The corrosion patterns indicate the possibility of ferrous contamination that usually occurs in fabrication areas when workers either grind mild steel close to stainless steel or use the same abrasives for both stainless steel and other metals.
• Post-weld treatment and post-fabrication restoration of the surface integrity was either not doe or not done properly.
Possible cause
There usually is no one single factor that would be the lone culprit when it comes to the failure of stainless steel products. It is rather a case of several small things going wrong. In this case we can confirm a good material choice, a non-problematic design and an installation environment that remains stable within the design parameters. We do find some serious flaws in the fabrication process that detracted from the materials ability to render proper service in these conditions. It is our opinion that that should the same material be used with the correct surface finish and fabricated to acceptable manufacturing standards, the items will give a cost-effective service life of possibly decades.
Summary and recommendations
Ferrous contamination in a high chloride environment support by rough surface finishes will lead to a serious pitting attack as experienced in this installation. Pitting is unpredictable and some pits form lateral cavities in the material, and it can spread over significant parts of the equipment. This type of metal degradation is not visible and, as such, very dangerous with unexpected early failure. We would therefore recommend that all the existing material be scrapped to ensure that there is no risk of hidden pitting in salvaged material. The manufacturer of the components should be given a full brief in terms of fabrication standards and requirements (Sassda can assist with this). In critical or special applications, it is advisable to only make use of fabricators and installers with a good track record with this type of work. Stainless steel is costly in terms of initial material cost, but Life Cycle Costing proves it to be unsurpassed as cost-effective solution. It is therefore important that the material is handled by knowledgeable and skilled persons.
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