Influence of Iron in Pickling Solution of Sulfuric Acid on its Activity H. BRUNCKOVÁ and Š. NIŽNÍK Institute of Experimental Metallurgy, Slovak Academy of Sciences, CS-043 53 Košice Received 15 April 1991 Parameters of deactivation of H2S04 pickling bath, resulting from the presence of iron ions, were determined on the basis of laboratory pickling tests of steel sheets prior to enamelling. The critical value of mass loss of sheet at pickling was determined by means of artificially inoculated pickling solutions increasingly saturated by carbonyl iron and in gradu^ly saturated solution by means of iron from steel sheets. Relationship between the increase ofsron amount in pickling solution and the amount of carbonyl iron was determined by mathematical regression analysis. Pretreatment of steel sheets prior to enamelling tion with iron ions. Analysis is based on experi­ is carried out by the classical chemical proce­ mental results obtained at pickling of deep-draw­ dure which includes an alkaline degreasing, pick­ ing cold rolled steel sheets intended for enamel­ ling in inorganic acids, neutralization, rinsing and ling. Ni-flashing [1, 2]. Pickling prior to enamelling fol­ lows the purpose to prepare the surface of sheets EXPERIMENTAL for "anchorage" of porcelain enamels in contrast to the classical pickling aimed at the removal of Cold rolled steel sheets produced by East FexOy oxides. For example, at the direct-on white Slovakian Ironworks (Košice) according to CSN enamelling process a mass loss from the sheet 411 310 with chemical components given in Ta­ 2 surface, amounting to 20—50 g m" , is required. ble 1 were used. Pickling efficiency tests were This process removes a layer 5—10 /xm thick [3]. performed on specimens of dimensions 50 mm x Minimally 90 % of pickled products are treated 100 mm and thickness 1 mm, intended for enam­ in inorganic acids, particularly in H2S04, HCl, and elling. HN03 or in the mixture of acids. It is obvious that Microstructure of material is ferritic, homoge­ the selection of the pickling bath has a substan­ neous, composed of polyhedral, very slightly elon­ tial influence. Increase of pickling intensity in gated grains of medium size 20 цт. Cementite H2S04 can be controlled by suitable setting of phase creates chains in the form of polyhedral bath temperature, since the volatility of the acid configurations in the direction of rolling. There are slightly increases with rising temperature [4]. On cavities among the cementite particles, created the other hand, it appears more reasonable to during the process of cold forming, as a result of control the pickling process in HCl by the change crushing of these hard phases. of its concentration on the grounds of sufficient The constant starting state of sheet specimen mass losses and the high volatility of this acid surfaces was achieved by two-stage degreasing [4]. Electron-microscopic examination indicates [5] procedure. The first stage consisted in prelimi­ that the sheets pickled in H2S04 have uniformly nary degreasing of specimens in organic solvents wrinkled surface with fine morphology. Pickling after which followed the galvanic degreasing in with H2S04 produces various crystallohydrates of electrolyte, containing 10 g of SYNALOD 3024 in 3 FeS04 (FeS04. H20, FeS04. 4H20, FeS04. 7H20) 1 dm of H20, lasting for 2 min at 6 V and 50 A according to the degree of saturation of solution m"2. SYNALOD is a white, crystalline, water-solu­ with iron ions [6]. ble substance containing metasilicates, carbon­ The presented paper concentrates on the analy­ ates, pyrophosphates, and detergents. sis of pickling activity and gradual wasting of Pickling efficiency tests were carried out using 3 H2S04 pickling bath in dependence on its satura- 1 dm of 1.13 M-H2S04 considered as stock solu­ tion in an apparatus with secured reproducible Table 1. Content of Elements in Steel Sheets oxygen content in solution. Pickling time amounted to 6 min at the temperature 60 °C. Mean mass Element С Mn Si Cu S P AI losses per 1 m2 area of sheet surface at pickling w/% ÖÖ4 Ö25 0.04 0.016 0.010 0.011 0.035 were calculated from three specimens. Chem. Papers 46 (3) 170-173 (1992) IRON IN PICKUNG SOLUTION RESULTS AND DISCUSSION of incipient oversaturation of solution with regard to Fe2+ concentration. Table 2, however, shows Influence of the Amount of Carbonyl Iron in that with regard to appreciably very continuous Acidic Solution on its Activity character of the change of mass loss values this method cannot be used for unambiguous deter­ Pickling activity of acid was determined by mination of the limit, beyond which the acid loses measurement of mass loss of sheet specimens its activity. and analytical determination of the increase of Pickling activity of acid was confirmed by quali­ iron amount in the set of artificially inoculated tative and quantitative determination of Fe con­ H2S04 solutions after pickling. The pickling centration in solutions after pickling. Determina­ solutions were prepared by addition of carbonyl tion of the total Fe content in the solutions 1 up iron in amounts 1,5, 10, 20, 22, 25, and 30 g into to 9 (Table 2) was carried out photometrically with 3 2+ 1 dm of 1.13 M-H2S04. KSCN, by oxidation of Fe ions by means of >>(Fe)/(g drn ) Fig. 1. Dependence of the difference of the increase of Fe concentration in the stock solution (1.13 M-H2S04) on the concentra­ tion of carbonyl Fe in the pickling bath. 3+ Mass loss measurements indicated that some (NH4)2S208 to Fe ions. With regard to the short decrease of values was recorded starting from pickling time (6 min) and secured reproducible the pickling solution containing carbonyl iron in oxygen content the presence of ferric ions is amount exceeding 20 g dm"3, suggesting the state negligible. Table 2. Results of Measurements of Fe Concentration after Pickling Pickling solution Solution after pickling n 0) PP (Fe) = pa(Fe) - pc(Fe) Ap = pg(Fe) - p^(Fe) n Pc(Fe) p(H S0 ) Pa(Fe) Absorbance g m"2 2 4 g dm g dm -3 3 3 g dm g dm" g dm" ^500 Standard 1 - - - 0.011 1 0 8.4 107.5 2.0 0.022 2.0 0 2 5 8.3 97.62 7.64 0.084 2.64 0.64 3 7.5 8.2 92.93 10.32 0.112 2.92 0.92 4 10 8.5 87.93 13.18 0.145 3.18 1.18 5 15 8.2 78.94 14.44 0.201 3.44 1.44 6 20 8.1 69.77 23.55 0.259 3.55 1.55 7 22 7.6 66.13 25.63 0.282 3.63 1.63 8 25 7.4 61.12 28.49 0.313 3.49 1.49 9 30 6.8 52.91 33.18 0.365 3.18 1.18 3 p(pickling 1.13 M-HjSOJ = 111 g dm" , pc(Fe) - concentration of carbonyl Fe in solution, pa(Fe) - total concentration of Fe after pickling, Pp(Fe) - values of the increase of Fe concentration in the nth solution, Ap - difference of the increase of Fe concentration compared to the first solution (pj = 2.0), со - area density of mass loss of sheets. Chem. Papers 46 (3) 170-173 (1992) 171 H. BRUNCKOVÁ, Š. NIŽNÍK Fig. 1 shows the typical curve of saturation of were subjected to successive pickling in 1 dm3 2+ solution with Fe ions described by the following of 1.13 M-H2S04 (the stock solution), for 6 min parabolic equation of the nonsymmetrical type each, at the temperature 70 °C. Constant condi­ tions were ensured by keeping the pickling bath 2 2 2 Ap = 15.3 x 10" pc(Fe) - 0.37 x 10" [pc(Fe)] in a calorimeter while appropriate instrumental arrangement prevented the evaporation of acid. where Ap/(g dm"3) is the difference of increments 1 dm3 of solution was used for pickling of ap­ of Fe concentration compared with the stock proximately 500 specimens. This experiment rep­ solution (1.13 M-H2S04) without Fe, pc(Fe) con­ resents a model of the pickling process under centration of carbonyl Fe in the pickling solution. practical conditions. The behaviour of the curve very distinctly points The relationship in Fig. 2 shows that the area to the change of activity of sulfuric acid induced density of mass loss (со) of specimen up to the by the presence of artificially introduced carbonyl concentration value of 1 g Fe in 1 dm3 of solution Fe. Linear dependence suggesting the stability of is somewhat higher than the subsequent first the pickling process extends up to the con­ plateau around the medium value of со = 10.4 g centration approx. 6.5 g of carbonyl Fe in 1 dm3 m~2 with the dispersion variance ±9.6 %, which of solution. Afterwards the pickling activity of acid extends up to the value of 7 g Fe in 1 dm3 of gradually diminishes up to the value approximately solution. Next plateau with slightly declining ten­ -3 3 pc(Fe) = 20 g dm . Inflexion point observed at dency terminates at p(Fe) = 18 g dm" and its this value represents the loss of active pickling average value of the area density of mass loss capabilities. This is demonstrated by the decrease equal to 6.8 g m~2 represents about 35 % de­ of mass loss of specimen. crease. j>(Fe)/(g dm ) Fig. 2. Dependence of the area density of mass loss of sheets on the amount of Fe dissolved in the stock solution. Results of measurements indicate that from the Accuracy of experimental results was verified by viewpoint of evaluation of the influence of dissolved analysis of the amount of dissolved Fe in used Fe on the capabilities of pickling solutions more stock solution by means of photometric and AAS accurate picture is obtained on the basis of depen­ methods.