Both sulfuric and hydrochloric acid are in common use for the pickling of steel to be galvanized, but hydrochloric acid offers several advantages and is recommended for use in new plants. There are several reasons, and some are outlined below:
- Energy consumption – As mentioned above, sulfuric acid must be kept at elevated temperature while hydrochloric is used at ambient temperature. ( hydrochloric must be 70F or above to pickle at an acceptable rate so that some heating may be required in the winter months )
- Operator Skill – Because of its nature, sulfuric acid, even when used with an inhibitor, will work its way under the surface rust or scale and attack the base metal of the article being pickled. On the other hand, hydrochloric with the proper inhibitor will remove rust or scale and not attack the base metal. This means that an article may be left in hydrochloric acid solution for extended times without damage and therefore is easier for an operator to use.
- Zinc Pickup – As mentioned above, sulfuric acid readily attacks the base metal and generally results in a rougher surface on the pickled article. This rough surface results in more ounces of zinc being used per square foot of surface area than the relatively smooth surface produced by hydrochloric acid pickling. Data exists in papers presented at American Galvanizers Association technical meetings, and in private communications to show that this increase in zinc consumption may range from 5 to 20% depending upon operator skill.
- Water Treatment Costs – In order to prevent contamination or the flux solution, articles pickled in sulfuric acid must be thoroughly rinsed before immersion with the attendant cost of treatment and disposal of the rinse water. With the use of hydrochloric acid, while rinsing is recommended, it does not have to be as thorough, and the flux bath can be easily adjusted chemically if carry over of acid results.
- Personnel Safety – While chemical burns are not frequent, sulfuric acid at 150F is definitely more hazardous than is hydrochloric at room temperature, a fact to which the writer can attest after several years of experience
Another much discussed disadvantage of hydrochloric is its effect upon buildings and equipment. This may be true in an unprotected steel structure, but if proper precautions are taken as in the proposed plant, the structure can be made resistant to this effect.