Behavior of Nickel-Copper Alloys in Sulfuric Acid

Monel 400 is commonly used in sulfuric acid based applications in reducing environments. So, this material is the best alternative to stainless steels and other materials when the sulfuric acid solutions are not highly oxidizing in nature. Monel 400 attains nominal corrosion rates in air-free sulfuric acid about 85% concentration at 30oC and about 60% concentration at 95oC. Even alloy 400 offers suitable corrosion resistance at the boiling temperature of about 20% sulfuric acid.

In saturated air the attack on Monel 400 increases, although in most applications the corrosion rate is equivalent to rates at air-free conditions because air saturation occurs rarely.

The rate at which air can be renovated at a quiet solution air interface is nominal in comparison to the rate at which oxygen is eradicated through corrosion. As a result, in a tank open in air, a large level of aeration would occur only at the liquid line, and if this level is constant, corrosion would occur at the fast rate at this level. Attention should be given to the nitrogen layer in these media. Most organic compounds containing proteins for example milk albumin, fats, sugars etc will interact with dissolved oxygen to eradicate from solution and may also contribute in the development of a more secured layer on the alloy surface.

In air-free sulfuric acid about 5 to 6% content, temperature has nominal influence, although in air saturated acid, the increasing temperature results into increased corrosion. The highest corrosion rate in air saturated media is noticed at 82oC. The corrosion rate decreases with increasing temperature until the boiling temperature when the corrosion rate is same as in air-free acid.

With the drastic increase in corrosion rate at high magnitudes of sulfuric acid, the limit of protection may be practical. Actually Monel 400 shows suitable resistance in the storage of 80% sulfuric acid at room temperature however it should not be used constantly at higher acid concentration before testing.

Alloy Monel 400 bar also offers god corrosion resistance to boiling sulfuric acid solutions about 15% concentration. Oxidizing salts like cupric and ferritic salts when dissolved in considerable magnitudes in sulfuric acid solutions increase the corrosivity of the solutions to the large scale. To show the corrosive effect of ferritic ion, lab tests were performed with Monel 400 in dilute, air saturated sulfuric acid solutions at 30oC in presence or absence of ferritic sulfate. Variations in ferric sulfate concentrations have also increased the corrosion rates more than variations in acid magnitudes.

Ferrous and cuprous based salts in application solution often do not have any effect on the corrosion rate of Monel 400 however it is crucial in dealing with solutions to prevent oxidation that would result into transforming these salts to higher valence form. Practically, it has been noticed that specific impurities will also prevent corrosion for example milk albumen, amines, ketones and meracptans prevent corrosion to some level.

By increasing the velocity of acid the corrosion rate of Monel alloy 400 also increases.