Steels alloy, pickling

Sulphuric acid is used to a very large extent for pickling low-alloy steels. The rate at which it removes the scale depends on (q) the porosity and number of cracks in the scale, (b) the relative amounts of wiistite, decomposed wiistite, magnetite and haematite in the scale, and (c) factors affecting the activity of the pickle. 

The furnace scales which form on alloy steels are thin, adherent, complex in composition, and more difficult to remove than scale from non-alloy steels. Several mixed acid pickles have been recommended for stainless steel, the type of pickle depending on the composition and thickness of the scale For lightly-scaled stainless steel, a nitric/hydrofluoric acid mixture is suitable, the ratio of the acids being varied to suit the type of scale. An increase in the ratio of hydrofluoric acid to nitric acid increases the whitening effect, but also increases the metal loss. Strict chemical control of this mixture is necessary, since it tends to pit the steel when the acid is nearing exhaustion. For heavy scale, two separate pickles are often used. The first conditions the scale and the second removes it. For example, a sulphuric/hydrochloric mixture is recommended as a scale conditioner on heavily scaled chromium steels, and a nitric/hydrochloric mixture for scale removal. A ferric sulphate/ hydrofluoric acid mixture has advantages over a nitric/hydrofluoric acid mixture in that the loss of metal is reduced and the pickling time is shorter, but strict chemical control of the bath is necessary. 

Most descaling and passivation processes for steels were developed prior to the widespread use of electrochemical techniques. As a result, a variety of visual and chemical tests are widely used for determining the surface cleanliness. Chemical tests have also been established to verify the presence of a robust oxide film on austenitic and ferritic stainlesses (8). These methods are very simple to conduct in a manufacturing environment, but they are qualitative in nature and rely strongly on the judgment of the inspector. Outside of the laboratory, electrochemical methods have not been widely used to evaluate cleanliness of carbon and alloy steels after pickling. Nevertheless, they are well suited for this purpose and have been examined in considerable detail in laboratory studies. 

Carbon steel and low alloy steels may be cleaned by electroetching or by pickling in a hydrochloric acid bath (8-12 wt%) at 40°C for 5-15 min. to strip the oxide from the surface. 

Alkaline permanganate pretreatment of steel for the removal of heat scale and smut prior to acid pickling results in faster descaling and reduced metal attack (see Metal surface treatments Metal treatments). Stainless steel alloys can also be cleaned by alkaline permanganate followed by pickling in nonoxidi2ing acids (260). 

Zinc and Zinc Alloys. Zinc metal is highly reactive in acid solutions such as sulfuric, hydrochloric, and nitric dissolving rapidly at acid concentrations normally used to pickle steel and aluminum. Dilute (1—4%) solutions of these acids can be used with caution to remove zinc oxides. 

Other Metals. Metals such as the austenitic series. Types 301—347, and the ferritic series. Types 409—446, of stainless steels may be enameled, as well as a number of other alloys (17). The metal preparation usually consists of degreasiag and grit blasting. Copper, gold, and silver are also enameled. These metals are usually prepared for appHcation by degreasiag. Copper is pickled usiag either a nitric acid [7697-37-2] or a sulfuric acid [7664-93-9] solution, followed by a dilute nitric acid dip. Silver may be pickled in hot dilute sulfuric acid followed by a dip in a nitric acid solution (18). 

Electrolytic Tinplate. Much of the tin mill product is made into electrolytic tinplate (ETP). A schematic of an ETP cross section is given in Figure 1. The steel strip is cleaned electrolytically in an alkaline bath to remove rolling lubricants and dirt, pickled in dilute mineral acid, usually with electric current applied to remove oxides, and plated with tin. It is then passed through a melting tower to melt and reflow the tin coating to form the shiny tin surface and the tin-iron alloy layer, chemically treated to stabilize the surface to prevent growth of tin oxide, and lubricated with a thin layer of synthetic oil. 

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