Click to download and read pdf
This article outlines a Sassda case study involving pitting corrosion of a stainless steel support railing system following an upgrade to 316L material, which was expected to be an improvement on the original 304 system. Sassda’s acting Executive Director Michel Basson reports.
Sassda was recently called upon to comment on visible corrosion and pit marking on the stainless steel railing system, installed in a biokinetics pool and necessary for safely supporting patients being treated in the water. Within six months of the pool being refurbished, the first visible signs of corrosion and pitting became apparent on the newly installed railings.
Although the pool is filled with a saltwater solution, the chloride and chlorine levels were within the recommended corrosion protection range of the grade 316L material used. The pool temperature is maintained between 32 and 34°C and the pool is in an enclosed environment, with a high roof enabling humidity levels to be kept low. The grade of stainless steel used has been confirmed by spectrometer testing to be the specified austenitic grade, 316L.
As seen from Figure 1, the railing components are submerged in the saltwater solution and never cleaned. However, the components are exposed to water being constantly circulated and therefore not oxygen deficient or stagnant. What is interesting about this investigation is the fact that the grade 316L railing system was installed to replace a grade 304 system which had been in use for four years before the first signs of discoloration and possible corrosion became evident. The replaced equipment was still available and spectrometer tests confirmed that grade 304 stainless steel had been used. Figure 2 shows the damage to the material surface caused by pitting corrosion. At the lower right, a very rough surface finish is visible, while on the right, the mounting to the pool structure reveals the onset of crevice corrosion. Also to be noted are the unsealed crevices, which lack post-weld treatment, and the tube surface finish, which shows an incorrect grain direction.
Failure of components is not usually caused by one incident or for one reason. It is common for failure to be attributed to a series of smaller, less critical factors and therefore important to investigate the following:
Material grade
As mentioned, the previous installation had been manufactured in grade 304 stainless steel and the new installation in grade 316. The owners of the pool reported good lifespan and performance from the original 304 stainless steel, while the new 316 grade showed signs of pitting as early as six months from start of use.
Since grade 316 contains molybdenum it is especially protected in high chloride environments, so the 316 should have performed better in this application. Technically, the original grade 304 should not have been suitable for the application since it lacks the specific protection against pitting corrosion offered by the molybdenum content in grade 316. In this installation, the opposite was found to be the case.
Material surface finish
Such applications would normally make use of very highly polished surfaces. Sassda would ordinarily not advise using any surface finish less than #600 grid in this scenario and, judging from the visual evidence, the finish on the new railing components is not up to requirement and was found to be as rough as #180 grid.
It was also noted that the finish was in the circumferential grain direction. When the components are installed horizontally, the grain direction points vertically and water that could possibly contain contaminants can drain off. If a component is installed vertically, the grain direction becomes horizontal and the small ridges impede the washing off to remove any contaminants or chloride deposits.
The visual evidence also pointed to the fact that the welding was not properly smoothed down as expected for this type of application. The equipment should have had at least the same shiny surface smoothness as the hand railing and similar structures we find at airports and public places.
Exposure to salt or harmful chemicals
Although the choice of grade 316 stainless steel was correct, the surface finish was not to the required standard. This could potentially limit the performance of the stainless steel and evidence points to this. Even with occasional spikes 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 addressed by the fact that the water circulating in the pool will prevent stagnant conditions. There are, therefore, 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 contributing factors.
Cleaning frequency and method
Since the equipment remains submerged, a standard cleaning regime should not be required. Any chloride build-up is washed away during use and/or by the circulating water. No additional cleaning regime need be in place and no additional chemicals need be used. Maintenance, or the lack thereof, cannot have been a contributing factor in this case.
Fabrication
Several fabrications and design issues came to the fore as part of the investigation. The following issues stand out:
- Inadequate surface finish should not be a real fabrication issue since the tube can be bought from distributors in the required finish. This case could be a result of ignorance regarding forms and finishes available.
- The weakest points in any assembly for use in corrosive conditions 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. The water in the crevice will be stagnant and the passive layer in this localised area will be compromised. Further to this, a secondary galvanic effect will occur in the crevice, with hydrogen breaking away from the water molecule and binding with chloride ions to form hydrochloric acid in the crevice. This is why corrosive by-products are so abundant at joints to 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 fully purged to ensure maximum weld integrity.
- Welding on this project seemed to have been done using stick electrodes. This gives rise to excessive heat in the welded area, which impacts negatively on the weld metallurgy as described earlier. Welding for this type of application needs to be much more sophisticated to control heat input and to protect the metallurgy of the joint.
- Some of the components had been exposed to extreme forms of corrosive attack over large continuous areas. The corrosion patterns indicated the possibility of ferrous contamination that usually occurs in fabrication areas where 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 done or not done properly.
- As mentioned previously, there is usually no one single factor that causes a stainless steel product to fail. Rather, it is a case of several small things going wrong. In this instance we could confirm good material choice, a non-problematic design and an installation environment that remains stable within the design parameters.
We did find some serious flaws in the fabrication process that detracted from the material’s ability to render proper service in these conditions. It is our opinion that, should the same material be used with the correct surface finish and fabricated to acceptable manufacturing standards, this 316L stainless steel railing system would give a cost-effective service life and possibly last for decades.
Recommendations
Since pitting is unpredictable and not clearly visible and, as such, dangerous with respect to unexpected early failure, Sassda recommends that all the existing material be scrapped to ensure there is no risk of hidden pitting in salvaged material.
The manufacturer of the new 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 make use of fabricators and installers with a good track record for this type of work. Stainless steel is costly in terms of initial material cost, but life cycle costing (LCC) proves it to be unsurpassed as a cost-effective solution. It is therefore important that the material is handled by knowledgeable and skilled persons.
