In the cleaning of boilers using an acid solution added with an inhibitor, sometimes occurred considerable corrosion of pinhole-like pitting type. This is largely considered to be due to Fe
3+ and Cu
2+, in the acid solution formed by the dissolution of scale and corrosion products during acid cleaning.
Laboratory test was carried on the corrosion rate and pitting formation of welded steel specimens immersed in 7% inhibited hydrochloric acid containing Fe
3+ and Cu
2+ at 70°C for 6hrs, and also on corrosion preventive effect of stannous chloride added in the same acid solution.
The results are summerized as follows:
(1) When inhibitor alone is used, corrosion rate increases in proportion to the concentration of Fe
3+ and Cu
2+ and pinhole-like pitting is caused at high concentration of Fe
3+ and Cu
2+.
(2) Addition of stannous chloride in conjunction with the inhibitor is very effective to reduce the corrosion rate and to prevent the formation of pitting.
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