Abstract
In the literature there are not much data available to describe the corrosion behavior of nickel alloys and special stainless steels in acids at high temperature, in particular above the boiling point. Therefore, a laboratory testing program was performed with the 6 % Mo stainless steel alloy 31 (UNS N08031), and the nickel alloys 59 (UNS N06059) and B-2 (UNS N10665) to obtain corrosion data in formic acid, acetic acid, phosphoric acid, polyphosphoric acid, p-toluene sulfonic acid and lactic acid at high temperatures up to 300 °C.
From the results it was concluded that alloy 59 was the best choice for formic and acetic acids at temperatures up to 300 °C. In phosphoric acid up to approx. 120 °C alloy 31 exhibited the best performance. At higher temperatures the acid becomes extremely corrosive and none of the investigated materials was resistant. In the diluted acid (1 %) alloy 31 was fully suitable up to at least 300 °C and alloys 59 and B-2 up to 200 °C. Alloy B-2 and alloy 59 showed the best behaviour in polyphosphoric acid at 200 °C. In 30 % concentrated p-toluene sulfonic acid at 200 °C all alloys tested were unsuitable; at 100 °C the corrosion rates were very low for both alloy 59 and alloy B-2 but slight local corrosion might occur. In much diluted acid (0.1 %) alloy 31 behaved well. In lactic acid at 180 °C increased corrosion underneath deposits was detected; the best performance was observed for alloy 59.
Further tests with lower acid concentrations and/or lower temperatures are underway.
