The scaling of the cooler will not only affect the heat exchange efficiency, but also make the scale layer thicker and thicker and cause serious local corrosion. Therefore, it is best to perform a single pickling on the scaled cooler during major maintenance.Pickling is mainly aimed at certain hard scales and metal corrosion oxidation products that are easily soluble in acid. Its effect on silicate scale and anhydrous calcium sulfate scale is limited.The acid used for pickling is mainly hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, and citric acid.Hydrochloric acid is most commonly used because hydrochloric acid is inexpensive and dissolves corrosion products and scale faster than sulfuric acid of the same concentration.Hydrochloric acid cannot be used for stainless steel equipment, but it is more appropriate to use 10% dilute nitric acid or 15% phosphoric acid for cleaning.
Acid dissolving scale will also corrode the steel itself, and hydrogen depolarization will occur. The H2 generated in the reaction will diffuse into the metal, causing hydrogen embrittlement in the equipment being washed. Therefore, corrosion inhibitors must be added during pickling to suppress acid corrosion of metals.If the pre-film treatment cannot be carried out in time after pickling, the fresh metal surface exposed after pickling is very lively, and it is easy to produce floating rust, which affects the pre-film effect.Therefore, it is necessary to neutralize with about 0.2% NaOH solution after pickling, then rinse with clean water, and then passivation treatment. The purpose is to keep the cleaned metal surface clean and free from rust.The passivation method is to immerse the washed equipment in the passivation liquid for more than 4 hours. If the immersion time is longer, the passivation effect is better.