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Preliminary Exploration on Corrosion of Boron Stainless

Author: ZuoQing
Tutor: HuZuo
School: Chongqing University
Course: Chemistry
Keywords: corrosion boron stainless alternating-current impedance Tafel weightlessness
CLC: TG142.71
Type: Master's thesis
Year: 2013
Downloads: 70
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Abstract


Hailed as the best steel, boron stainless is in ever-increasing demand in industrywith numerous advantages such as high hardness, heat-resistant, anti-corrosion, largeneutron absorption cross-section and extraordinary ductility along with the developmentof nuclear power industry and the automotive industry. Thus, with very few research oncorrosion, it is of great significance to study the corrosion in boron stainless for betteruse in industry on account of the fact that corrosion may give rise to economic lossesand certain accidents.Corrosion electrochemical phenomenon of boron stainless in boric acid andhydrochloric acid medium has been investigated by immersion test method, Tafel linearextrapolation method, alternating-current impedance method and circuit study matchedby software Zview to obtain the parameters such as self corrosion potential, corrosioncurrent density, polarization resistance and corrosion rate of different concentrations,temperatures and pH values. The corrosion appearance and material ingredients havebeen studied by scanning electron microscopy （SEM） and electronic data switching（EDS） respectively, while weight loss experiment were put into effect to caculate thecorrosion rate and analyze the corrosion mechanism of boron stainless. The mainconclusions are as follows:①The self corrosion potential of boron stainless steel is very small along with theincreasing of the boric acid concentration as the value of it decreases with the increasingof boric acid concentration, while on the contrary, the polarization resistance increases.The corrosion rate of boron stainless remains to be small when changing thetemperature, which indicates that boric acid has little effect on the corrosivephenomenon of boron stainless steel, and is just the very fact that boron stainless steelcan be used together with boric acid in the nuclear power industry. According to theimmersion test of increasing the immersion time, the obtained weight loss increased, forwhich oxygen corrosion produced during immersion.②The corrosion weight loss increased with the increasing of immersion time inhydrochloric acid solution, while with the increasing of hydrochloric acid concentrationat room temperature, a negative offset emerged in the corrosion potential, corrosioncurrent density increased to0.2032mA/cm2from0.032530mA/cm2, linear polarizationresistance dropped by585.9to96.4, mass transfer resistance Rct reduced to86.71 from438.8and the corrosion rate increased to4.6431mA/cm2from0.7433mA/cm2,which indicates that the concentration increasing of H+and Cl-can accelerate thedissolution of oxide film Cr2O3and B2O3on boron stainless steel surface to make surethe corrosion of the substrate subject. And corrosion would be aggravated on account ofthe presence of Cl-in the sotches.The corrosion potential changed from-385mV to-340mV when immersed boronstainless steel for48h, while the corrosion current density decreases to-0.00469mA/cm2from0.2032mA/cm2and Rct increased for seven times to642.2from86.71, Which indicates that the corrosion rate of the matrix were slowed down as thelarge amount of corrosion products deposited on the surface along with immersing time.The corrosion current density of boron stainless steel increased to0.3111mA/cm2from0.1307mA/cm2when the solution temperature rised to50°C from roomtemperature, while the reaction rate of high temperature is three times larger than that ofroom temperature and the polarization resistance is half of that at room temperature,which demonstrates the fact that the moving rate of ions in solution accelerated for theincreasing of the temperature and consequently promotes the movement of the reactantsand the corrosion rate.③The corrosion weight loss in boron stainless increased slowly with the extensionof corrosion time in simulated acid rain. Polarization resistance in simulated acid rainsolution of pH=6.0is106times and three times larger than those in solutions ofpH=2.0and pH=4.0respectively. While the arc radius of capacitive reactance increaseswith the increase of the pH value, for which the corrosion current density of pH=2.0is29times and87times bigger than those of pH=4.0and pH=6.0respectively. The resultsindicate that severe corrosion occurred to boron stainless in solutions of pH=2.0, andthat is relatively lighter in solution of pH=4.0, while in the solution of pH=6.0, a largeamount of oxide film generate on the surface of boron stainless, which is just the veryreason why corrosion rate decreases with the increase of pH values.Polarization resistance of the boron stainless reduced to231.7from306whilecurrent density increased to0.1363mA/cm2from0.1006mA/cm2with the increasingtemperature in the solution of pH=2.0, which indicates that the increasing of corrosionrate can be the evidence to illustrate the fact that temperature increasing can acceleratethe corrosion of boron stainless steel.④According to the SEM image, the main forms of corrosion in boron stainless inboric acid, hydrochloric acid and simulated acid rain are intergranular corrosion and pitting in the immersion experiments, which is due to the fact that the corrosion ismainly determined by the element of Cr, while the addition of boron can make stainlesseasy to generate chromium depleted zone and prone to generate intergranular corrosionand pitting.⑤The oxidation film Cr2O3from the surface of stainless steel can be dissolved inacid and the solution with other corrosive anions such as Cl-, and then the stainless steelsubstrate can be hydrolyzed as the lengthen of immersion time, thus, the corrosion instainless steel may occur.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metallic materials > Steel > Special performance of steels > Stainless steel, acid-resistant steel
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