Instability of stainless steel reference weights due to corrosion phenomena

Abstract

Problems of instability of the mass of stainless steel reference weights may find an explanation in the long term corrosion phenomena which may occur on their surfaces. It has to be considered that reference weights, although maintained under controlled atmosphere, may sometimes come in contact with liquids, e.g. for cleaning operations, for density determinations, etc.. An investigation on the effect of contact with liquids on austenitic stainless steel weights has been carried out by means of immersion of AISI type 304 specimens in bidistilled water, at room temperature for three years, in boiling water and in boiling alcohol\\ether from few minutes to 24 h. A significant amount of metals was dissolved in all cases; the dissolution rate depends strongly on the temperature. The highest weight loss was found with specimens immersed in bidistilled water at room temperature: a decrease in mass of 2.2 μg\\cm 2 after the first year of immersion, corresponding to a corrosion rate of 1.2 · 10 −4 μm\\year, and a decrease of lower entity after the second year; in the third year no further variation was observed. The total mass variations caused by immersion in boiling water are similar to the previous ones, but they occur very rapidly. In organic solvents the weight loss is smaller. Theanalysis of the solutions, performed by means of a plasma source mass spectrometer, ICP-MS, revealed a preferential dissolution of Fe and to a lower extent of Ni, dependent on the dissolution conditions (solvent, time and temperature), and a noteworthy amount of dissolved Mn, which may come mostly from inclusions.