Reducing energy costs and improving the ecological purity of the materials remain the most problematic issues of degreasing surfaces. As practice shows, the existing degreasing water compositions at low temperatures (25-30 ° C) do not not provide the high quality of the surface cleaning. Decent results are likely to be achieved only with the use of organic solvents and emulsion formulations, but it negatively affects the environment. The contradiction is removed to a certain extent in the application of thermal degreasing methods, but it is not always a panacea for certain financial reasons. Such options as the use of compositions which do not require subsequent washing (for example, BOC-5) or allow discharging wash water without pre-cleaning can be considered, but hardly possible to resort to.
Scale, rust and other oxides are often removed from the metal surface by etching in solutions of acids, acid salts or alkalis. Cleaning the surface with etching method is reduced to the dissolution of the surface layer of oxides and metal recovery of oxide compounds and the separation of hydrogen generated. Etching products are subjected to pre-purified from mechanical impurities and fat.
Etching in ferrous metals
As etchants for ferrous metals the most commonly used materials are sulfuric, hydrochloric and phosphoric acid with various additives. In acid etching the kinetic curve to reduce the mass of the sample can be divided into three periods: dissolving oxides, the appearance of the anode surface areas, accompanied by the simultaneous dissolution of oxide and metal and dissolution of metal.
Sulfuric acid reacts more actively to iron than with its anhydrous oxides (by etching dissolves no more than 20% of the scale). Getting into the pores and cracks of scale, the acid dissolves the surface layer of the metal and thereby violates its relationship with the scale. Therefore, in the etching sulfuric acid is associated with the formation of a significant amount of sludge hydrogenation steel. The metal surface as a result of etching can have a highly uneven relief. The most frequently used acid concentrations vary from 150 to 200 g/l, the etching process is carried out at 50-80°C.
The hydrochloric acid descaling occurs primarily as a result of dissolution (weight reduction of scale is 50% or more). Consequently, the metal surface after etching in hydrochloric acid is smoother than in sulfuric one: the degree of sludge formation is also reduced. Etching in hydrochloric acid is conducted at temperatures of 20-60°C, optimum acid concentration of 100-150 g/l. The use of hydrochloric acid, however, is less convenient and economical than sulfuric acid. Therefore, etching oftentimes implies using sulfuric acid and mixtures of sulfuric acid with hydrochloric acid or sodium chloride. The presence of the latter reduces the corrosion rate of iron without affecting the dissolution rate of an oxide. An even more effective in this respect is the introduction of the composition of the etching solution of corrosion inhibitors (additives) represented by methenamine, and others.
A way more cost-effective technology, demanding considerable engineering and financial efforts, but paying off in the long run, can be ultrasonic cleaning. Pioneered by the industry innovators like Hilsonic, a far-famed ultrasonic cleaner equipment manufacturer, has recently presented a cost-effectiveness report on projects implemented in 2015, boasting impressive financial feasibility ratings.