The Effect Of Pickling Accelerating Ingredients On Carbon Steel Oxide Scales

In thermal processing of carbon steel, for exposed to air in the high temperature,oxide scale is generated on the surface, which affected the subsequent processing andindustrial production. Hydrochloric acid pickling is generally used in removal the scale,but exists something such as over-corrosion, underpickling. Proper pickling accelerativeadditive can increase the pickling rate and reduce the iron loss. Currently there are variouspickling accelerative additive, and most of them contain sulfur or phosphorus, which playthe promoting effect. There are little research on the mechanism of acid accelerating andthe mechanism is not clear, it is very necessary to study the effect of pickling acceleratingingredients during pickling process, for control the pickling time, reduce acid consumptionand environmental pollution.Because the structure of the scale has important effect on the surface quality of steeland the following process, the morphology and composition of scale was observed andanalyzed. Then through a lot of analysis in pickling accelerating ingredients, three picklingaccelerating ingredients were selected, sulfur-containing organic acid, sulfur-containinginorganic salt, phosphorous-containing inorganic salt. Through pickling time of scaleremoval and a series of electrochemical measurements such as potential derivative firstzero method, electrochemical impedance spectroscopy, potentiodynamic scanning,catholic polarization, potentiostatic polarization, Mott-Schottky plots analysis, the effectof pickling accelerating ingredients on the electrochemical behaviors of carbon steel oxideduring pickling process were investigated.The morphology and composition of scale show that the thickness of scale is between6-8μm, multi-layer and closely combined with Fe matrix, but there are many tiny cracks.The results of pickling time of scale removal and potential derivative first zero methodindicate that three substances all have a better role in promoting removal of scale, thepickling termination time decreases to16.8s, the promote efficiency is up to54.84%. Theelectrochemical impedance spectroscopy analysis find when added the three compoundsindependently, it can enhance the diffusion of ions, the charge transfer resistance became smaller, decreased the corrosion resistance of scale, the promote efficiency is up to86.48%. Through potentiodynamic scanning analysis, it reveals that the picklingaccelerating ingredients mainly affect the cathodic reaction of carbon steel scale, thecorrosion potential became negative shift relative to the blank, and the polarization curvesmove to the higher current direction. The catholic polarizations show that threecompounds induce low hydrogen evolution over-potential, decrease the activation energyof electrode reaction, and the hydrogen evolution process become easier to carry. Thepotentiostatic polarizations find that the sulfur-containing organic acid and thesulfur-containing inorganic salt can speed up the hydrogen evolution reaction and promotethe diffusion of H+, increase hydrogen generation, and the phosphorous-containinginorganic salt mainly cause the hydrogen evolution reaction more quickly. Using theMott-Schottky plots analysis, results show that the carbon steel scale is with P-typesemiconductor characteristics, and the carrier is mainly for holes transmission in the scalelayer. After adding three additives, the concentration of carrier become larger, which isbeneficial to the generation and transmission of oxygen holes and metal ions holes, andpromote the corrosion of the scale.