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Influence of different HCl concentrations on Monel 400 and Nickel

phosphoric acid plants

The influence of hydrochloric acid concentration on the corrosion rates of Monel 400 and nickel in air-free and air-saturated solutions at 30oC. It will be noted that the corrosion rates of Monel 400 and nickel at room temperature are about the same 10% acid concentration. In this concentration limit, the corrosion rates of both materials in air-free solutions are usually below 0.01 ipy. The rates for materials are significantly enhanced by saturating the solutions with air.

Above 15% acid concentration, the corrosion rates of Monel 400 increase steeply. Usually the applications of Monel 400 at room temperature are suitable below 10% acid concentration in aerated solutions and below 20% in air-free solutions.

Nickel offers superior performance to Monel 400 in HCl concentrations above 20% at room temperature, probably because nickel chloride is less soluble than cuprous or cupric chloride in this acid concentration range. The solubilites of these chlorides in HCl solutions are evaluated. Usually nickel finds suitable applications when acid concentration is up to 30% both in aerated and unaerated conditions at room temperature. The corrosion rate was found maximum to 574 mdd at 5% concentration, then decreased to 500 mdd, at 10% and did not increase even at higher concentrations to 30%. When acid concentration is 25%, a steep drop in corrosion rate is noticed. Various test samples in different solutions of this concentration, evaluated closely. it has been considered prudent to disregard this low point as on the commercial level, it may be tough to maintain right this concentration. The use of nickel with concentrated hydrochloric acid solutions I limited and there is nominal information available to confirm these test outcomes. If the behavior of nickel is resulted by the comparatively low solubility of its corrosion products, care should be taken while applying these data to operate in concentrated acid where high speed occur.

Solution Temperature

With increase in temperature, the corrosion rate of alloy 400 and Nickel increases in hydrochloric acid solution. However Monel 400 shows superior performance to Nickel. When Monel 400 is used in unaerated 5% acid concentration it offers significant resistance up to 75oC whilst nickel is suitable up to 55oC. Applications of these alloys specifically Monel 400 wire uses at higher temperatures and in lower concentrations are well known. In several processes which involve the production of hydrochloric acid, hydrolysis of chlorides or chlorinated solvents, acid concentrations are below 0.5% , these alloys show suitable performance up to 300 to 400oF.

Performance of Monel 400 and Nickel in air- saturated HCl at temperatures above room level are often constrained to acid concentrations below 3-4%. Monel 400 offers successful performance in handling aerated acid concentration 2% up to 120oF and 1% acid concentration up to 180oF.

The outcomes of lab corrosion analyses of Monel 400 and nickel in boiling HCl show that monel 400 offers successful performance up to 1% acid concentration or lower and its performance is superior to Nickel. In the boiling atmosphere, ebullition sweeps the solution free of air to prevent aeration of acid.