Corrosion of Tinned Sheets during Transportation-A Case Study

At buyer's end complaints regarding black stains/spots on the tinned sheet transported by a Tin Sheet Manufa- cturing Co. were received very frequently.The problem was investigated thoroughly. In general, the tinned sheets are stacked one above the other and in between Di-octyl-seb-acate(DOS)is applied which acts as an inhibitor as well as the lubricant. Normally, 1000 sheets are stacked in bun-dle, strapped crosswise and then transported to different parts of the country. On examination of the whole system, it was observed that the sheets were packed loosely, resul-ting in fretting corrosion due to uneven load on the sheets and the relative movement of the sheets. Further, the DOD was found to be acidic in nature. No fault in electro-tinning process was observed.

The potentiodynamic polarization studies on the untreated and treated sheets with DOS and DOS containing stearic acid and sodium nitrite were also conducted in 3.5% NaCI solution. The OCP was found to be more negative in comparison to be treated sheets and the corrosion rate
of the treated sheets was also on the higher side. Photographs show that there is dissolusion of tine coating associated with fret. The data are discussed in details. Recommendation for the solution of the problem are given.