Corrosion performance of Incoloy 825 and Monel 400 in sulfuric acid

The corrosion resistance of precipitation hardening stainless steels is based on their structure and chemistry. Aging causes chromium carbide precipitation and reduces corrosion resistance. There are various alloys in this classification.

As end grain and side grain corrosion occurred to a larger extent on the bar stock than on the sheet samples, the corrosion rates on the bar samples were much higher than on the sheet samples.

Manganese and nitrogen have austenite stabilizers and in durations of short nickel supply, these elements have replaced a part of nickel in austenitic stainless steels. Normally, the corrosion resistance of SS 316, 216L is superior to SS 316, SS 316l. Although in dilute sulfuric acid, nickel is an essential alloying element and the corrosion resistance of SS 216 is lower to SS 316.

Besides to stabilizing austenite, manganese and nitrogen reinforce austenitic stainless steels and enhance resistance to abrasion and galling.

Incoloy 825 has outstanding resistance to sulfuric acid, particularly in the concentration range of 40%. It provides suitable performance in sulfuric solutions to 40% at atmospheric boiling temperature, to 78% concentration at 80oC and in all concentrations of the acid up to 65oC or 150oF. It is stabilized against carbide precipitation by inclusion of titanium so that it can be used in the as-welded condition.

Corrosion rate of Incoloy 825 is independent of specific heat of alloy or if the corrosion analyses were conducted in technical grade. The corrosion rate was based on the type of condenser used for the test equipment. Metal ions exist in acid from dissolution of the stainless steel act as oxidizing agents and lower the corrosion rate by a factor of about 10. This prevention was independent of the concentration of dissolved stainless steel in the range of 5 – 50 g/l of dissolved stainless steel. This influence was higher than that received by bubbling air in the solution, feasibily due to the limited solubility of oxygen in the boiling acid solution. It also shows that prevention could also be achieved by anodic protection.

Corrosion rates were about the same in liquid and vapour on samples of 1/4^th inch thick plate welded by the Tungsten insert-gas process.

Use Monel 400 bar for handling sulfuric acid is popular in reducing conditions. It provides an alternative to stainless steels and other alloys of similar behaviour when the sulfuric acid solutions are not firmly oxidizing. Alloy 400 attains low corrosion rates in air-free sulfuric acid up to 85% concentration at 30oC and about 60% concentration at 95oC. At the boiling temperature, Monel 400 has supreme resistance to 20% sulfuric acid concentrations are nominal.

While air saturation speeds up the corrosion of Monel 400 in various applications the corrosion rate will be approximate to the air-free rates as continuous air saturation is rare.

The rate at which air can be reused at a quite solution air interface is nominally compared to the rate at which oxygen can be eliminated by corrosion. As a result, in an open tank, a high level of aeration would occur only at the liquid line.