Evidence of corrosion in a number of tanks used for transporting chemicals prompted Industrial Member Consani Engineering (Pty) Ltd to contact TWI to investigate the cause of the unexpected attack.
The tanks in question held thiocyanate solutions and were between six and nine months old. Approximately 6m in length, 2.6m in diameter, and with a wall thickness of 4.8mm, they had been fabricated mainly from a 316 Ti grade of stainless steel and seam welded using plasma arc. Manual metal arc (MMA) welding had been used for the fittings.
At short notice, TWI carried out on-site metallurgical examination of the tanks in Belgium followed by chemical analysis in the laboratories at Abington.
Test results showed the attack to be pitting corrosion - local perforation of the passive film on the stainless steel with ensuing attack extending into the body of the material. The pits were, in the majority, located within the welded seams, although these welds were essentially sound. Ferroxyl testing indicated there to be negligible iron contamination of the surface.
The development of pitting corrosion on stainless steels is normally associated with an environment containing chloride ions, but in this case tests gave no indication of the presence of chlorides within the corrosion pits.
Most data sources indicate that austenitic stainless steels will resist attack in thiocyanate media over a wide range of concentration and temperature. However, previous information shows that pitting may occur in a restricted electrochemical potential range. The corrosion attack depends upon the specific impurities present and the redox potential attained.
In this case, the problem was associated with the batch of thiocyanate handled rather than with any material or fabrication defects.