Investigation into failing welds at a dairy highlight the importance of understanding your stainless steel
In August this year, Sassda visited a KwaZulu-Natal based dairy to examine pitting associated with welds in a newly installed holding coil. The farm was concerned that despite assurances that stainless steel was the best material, the coil was degraded.
At the site inspection it was established that the welding quality was not good due to space constraints and the purging during installation was probably inadequate due to poor procedures.
It was discovered that after installation, the holding coil was filled with a 0.5% solution of peracetic acid at a pH of 4.4. The chloride concentration was 26 ppm and the fluoride concentration was 2 ppm.
The solution was left in the holding coil for two to three weeks.
The peracetic acid manufacturers claim that this acid is stabilised and does not decompose to release oxygen. Dilution hydrolyses the peracetic acid and alters the equilibrium between peracetic acid, acetic acid, hydrogen peroxide and water. It is therefore unlikely that the diluted peracetic acid would replenish oxygen in stagnant water. Pitting initiated during this two to three week period.
From a hygiene cleanability point of view, weld quality is critical to avoid crevices or gaps, where micro-organisms can be trapped and multiply.
Welds should be smooth and free from cauliflower, caused by air contamination inside the pipe, and other weld defects such as lack of fusion.
Clearly, there was ample evidence that there was a lack of purging and the general weld quality was poor. In addition, the poor welding and stop-starts resulted in areas with higher heat inputs and thicker and darker weld discolourations with consequently lower pitting resistance.
These areas were most at risk.
From a corrosion point of view, this lack of adequate purging resulted in weld discolouration with a compromised pitting resistance. There is normally no need for pickling and passivation but in this case there was insufficient purging (weld purge less than 5 on AWS D18.1/D18.2).
To restore the corrosion resistance, this oxide and the underlying chromium depleted layer needed to be removed by pickling. Passivation was not necessary and would happen spontaneously. Pickling would not solve the potential hygiene cleanability problems caused by the other weld defects.
The chloride levels in the plant seem to be acceptable from a corrosion point of view. However, this is assuming that the 304 is passivated and that this passivation is maintained. At very low levels of oxygen in the water, passivity cannot be maintained. This is often seen in stagnant conditions and for stainless steels, an increased flow rate usually promotes the passivation process as access of oxygen is facilitated.
The two to three week stagnant conditions would thus not have been helpful in establishing a passive film at the welds and even where there was a passive film, depassivation was possible. The low pH contributed to the susceptibility to pitting corrosion. Stagnant conditions should always be avoided.
