Results of experiments indicate a new class of pickling inhibitors has been developed which gives excellent metal surface protection against sulfuric acid attack without altering the rate of acid attack against the oxide appreciably and which shows also excellent adhesion to the metal surface so that "after-treatment" may be delayed several days without re-rusting of the metal.
The article contains a brief review of the properties of inorganic inhibitors, metallic salts having a relatively high overvoltage; and organic inhibitors, hydrocarbon - nitrogen - oxygen - sulfur compounds which adsorb on the cathode area and thus protect metal.
Emulphor STH, a wetting agent, is an effective inhibitor although inhibitors in this class generally have not been effective in giving more than brief protection against reoxidation of the pickled surface. Two types of steel were used to indicate effect of composition on inhibitors: SAE 1010 and Type 8617, HTS 3121, Spec. AN-QQS-676. Curves show increased inhibitor concentration improves percent retardation but not proportionately. Minimum effective inhibitor concentration varies with steel composition. A method of calculating percent of retardation is given.
Authors indicate irregularities in rate of attack are shown better by hydrogen evolution method, although slight errors are experienced at start. Because industrial pickling often is done at elevated temperatures weight loss experiments were carried out at 62-65 degrees C with slow stirring. To duplicate industrial conditions experiments also were conducted with rusted specimens. Three tests were made to determine the effectiveness of STH inhibition against rerusting. The STH film can be removed readily with dilute alkaline solution.
Conclusions are that Emulphor STH is characteristic of a new class of organic inhibitors which combine excellent inhibiting properties with a sustained rust-preventng effect on highly-active freshly pickled surfaces.
