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Artigo_2002-MR-Falleiros-Correlation_between_Corrosion_Potential_and_Pitting_Potential_for_AISI_304_in_3_5_NaCl.pdf

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Correlation between Corrosion Potential and Pitting Potential for AISI 304L Austenitic Stainless Steel in 3.5% NaCl Aqueous Solution Neusa Alonso-Falleiros, Stephan Wolynec* PMT/EPUSP - Departamento de Engenharia Metalúrgica e de Materiais da Escola Politécnica da Universidade de São Paulo, Av. Prof. Mello Moraes, 2463, Cidade Universitária, 05508-900 São Paulo - SP Received: September 17, 2001; Revised: December 29, 2001 We investigated the effect of surface finish of two AISI 304L (UNS S30403
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  Correlation between Corrosion Potential and Pitting Potential forAISI 304L Austenitic Stainless Steel in 3.5% NaCl Aqueous Solution  Neusa Alonso-Falleiros, Stephan Wolynec* PMT/EPUSP - Departamento de Engenharia Metalúrgica e de Materiais da EscolaPolitécnica da Universidade de São Paulo, Av. Prof. Mello Moraes, 2463, CidadeUniversitária, 05508-900 São Paulo - SP Received: September 17, 2001; Revised: December 29, 2001We investigated the effect of surface finish of two AISI 304L (UNS S30403) stainless steels onthe corrosion potential (  E  corr ) in 3.5% NaCl aqueous solution and its value was compared with thepitting potential (  E  p ) value and the type of anodic potentiodynamic curve obtained for determinationof  E  p  in this solution. Five different surface finishes were examined.  E  corr  and its standard deviationare strongly affected by the type of surface finish. Moreover, there are evidences of a linearcorrelation between  E  corr  and  E  p , as well as between the percentage of anodic curves with awell-defined pitting potential and the uncertainty in the determination of  E  corr . Keywords:  pitting corrosion,    pitting potential , corrosion potential, austenitic stainlesssteel 1. Introduction Passive metals may become susceptible to pitting cor-rosion when exposed to solutions having a critical contentof aggressive ions such as ion chloride. This type of corro-sion is potential-dependent and its occurrence is observedonly above the  pitting potential ,  E  p , which can be used todifferentiate the resistance to pitting corrosion of differentmetal/electrolyte systems 1 . The  E  p  value can be determinedelectrochemically using both potentiostatic or potentiody-namic techniques 1 .In the potentiodynamic technique an anodic polariza-tion curve is obtained using a constant potential scanningrate. The  E  p  value is the potential at which a sudden increasein the current is observed. However, for a given metal(considered as having a given chemical composition and agiven microstructure) the  E  p  value obtained by this tech-nique depends on the following testing variables: the po-tential scanning rate 2-6 , the surface finish of the sample 7-12 ,the aeration conditions of the electrolyte 13 , the immersiontime prior to the test 6,8 , and the geometry of the sample 14 .Moreover, the obtained anodic curve does not always yielda precise measurement of  E  p . Very often, instead of asudden increase in current density at pitting potential (nor-mal curve - type N), this increase is slow and  E  p  is not welldefined (curve with not defined  E  p  - type NDP).In a previous work  8  it was found that the incidence of type NDP curves is strongly affected by surface finish. Itwas also observed 15  that the surface finish has a significanteffect upon the corrosion potential, E  corr , of the metal in thetesting solution, suggesting that there could be a correlationbetween this potential and the pitting potential. Similarobservation was made by Compère et al. 16  who found thaton average both the corrosion potential (open-circuit po-tential) and the pitting potential of 316L (UNS S31603)stainless steel increase with the immersion time in seawater. Moreover, they also found that a corrosion potentialof around 0 mV (SCE) to 65 mV (SCE) plays a significantrole in enhancing pitting corrosion resistance due to amodification of the passive film with an enrichment inchromium in the inner layer.In present work the influence of surface finish on thecorrosion potential  E  corr  of two austenitic stainless steels in3.5% NaCl solution was investigated. The obtained valuesof  E  corr  are analyzed taking into account the type of theresulting anodic polarization curve for  E  p  determination, aswell as the  E  p  value. 2. Experimental The chemical composition of the two austenitic stain-less steels is presented in Table 1, along with the standard Materials Research, Vol. 5, No. 1, 77-84, 2002.© 2002 *e-mail: swolynec@usp.brPaper presented at International Congress on Metallurgy and MaterialsTechnology - ABM, São Paulo, October 1994.  composition of AISI 304L (UNS S30403) steel 17 . It can beobserved that the Ni content of 304LL steel is considerablyin excess of the maximum value specified for AISI 304Lsteel. On the other hand, the C, P, S, Si, Mn and N contentsin 304LL steel are lower than in 304L steel and the second L is used to indicate this condition.All samples were submitted to solubilizing anneal at1050 º C for 40 min, quenched in water, pickled with 25%HNO 3  + 8% HF at 60 º C, ground with silicon carbide paperup to the grade 600, and passivated in 20% HNO 3 solutionat 35 º C for 60 min. Afterwards they were mounted inthermosetting plastic. The samples were passivated toavoid crevice corrosion at the thermosetting plastic/metalinterface during the electrochemical tests. The tests wereperformed in a 3.5% NaCl naturally aerated solution, pre-pared with analytical grade reagent and distilled water, at(23 ±  1) º C.The electrochemical tests were performed with aPrinceton Applied Research potentiostat, model PAR 273,using the saturated calomel electrode (SCE) as the refer-ence electrode and a graphite rod as the counter electrode.The pitting potential was determined by the potentiody-namic technique using a scanning rate of 1 mV/s. Thescanning was always initiated after 5 min immersion time,starting from corrosion potential and finishing when thecurrent density was at least 5000 µ A/cm 2 .The corrosion potential was measured at the momentthe scanning was initiated, that is, after 5 min immersion intesting solution. For some surface finishes the corrosionpotential was also recorded as a function of time for aperiod of 96 h.Five different surface finishes were used, namely:S1: grade 600 silicon carbide paper grinding immedi-ately before the immersion in testing solution;S2: grade 600 silicon carbide paper grinding followedby an immersion in distilled and deionized water for 88 h;S3: grade 600 silicon carbide paper grinding followedby an exposure to air in a desiccator for 88 h;S4: grade 600 silicon carbide paper grinding followedby pickling in 30% H 2 SO 4 , at 35 º C for 30 min, waterrinsing and drying;S5: grade 600 silicon carbide paper grinding followedby passivation in 20% HNO 3 , at 35 º C for 60 min, waterrinsing and drying. 3. Results In Figs.   1   and 2   the values of  E  corr  obtained after 5 minimmersion in 3.5% NaCl solution for the different surfacefinishes are shown with the corresponding  E  p  values for304L and 304LL steels, respectively. Both  E  corr  and  E  p values are averages of at least five and up to 20 measure-ments, and the error bars are their standard deviations.The data in these figures indicate that when a givensurface finish determines an increase or decrease in thepitting potential value, the corresponding corrosion poten-tial value is also increased or decreased. This can be visu-alized in Fig. 3, where  E  p  is plotted against  E  corr . The solidlines are the result of a linear fit to the data, excluding theS2 samples because their  E  corr  does not follow the sametrend as  E  p . The correlation coefficients r  2  obtained are 0.88for the 304L steel and 0.95 for the 304LL steel. The t  distribution significance test shows that the probability of a linear correlation between  E  p  and  E  corr  is 95% for 304Lsteel and 97.5% for 304LL steel. 78Alonso-Falleiros & Wolynec  Materials Research Table 1.  Chemical composition of steels . Element(weight %)SteelStandard compositionof AISI 304L 17 304L304LLC0.0140.0100.03 maxP0.0350.0030.045 maxS0.0210.0040.030 maxMo0.30--Cr18.618.518.0-20.0Ni10.3015.08.0-12.0Si0.45< 0.011.00 maxMn1.76< 0.012.00 maxN0.0320.0202-Cu0.21-- Figure 1.  Pitting potential  E  p  and corrosion potential  E  corr  as a functionof the surface finish for 304L steel.  A correlation was also observed between the percentageof type N curves obtained and the standard deviation of thecorresponding corrosion potential. This correlation is pre-sented in Fig. 4, which shows that when the standarddeviation of  E  corr  is low the probability of getting a type Ncurve, i.e. , with a well defined pitting potential, is high. Thecorrelation coefficient r  2  for this relationship is equal to0.8127. Using the test of significance for this correlation,it was found that for a probability of 95% the confidenceinterval for r is 0.68 to 0.98, which assures that there is areasonable correlation between these two variables.In order to get information on how  E  corr  varies withtime, its value was recorded during 96 h for S1, S3, S4 andS5 surface finishes. Typical records are given on Figs. 5 to10.For the 304L steel with S1 surface finish the  E  corr increases with time as shown in Fig. 5, until a value at whichthis potential stabilizes. 304LL steel behaves in a similarmanner. The time to reach the stabilized value varies from8 to 36 h.For the S3 surface finish the initial value of  E  corr  isconsiderably high but, as shown in Fig. 6 for 304LL steel,it decreases fast until a minimum value is reached, and then Vol. 5, No. 1, 2002Correlation between Corrosion Potential and Pitting Potential79 Figure 3.  Correlation between pitting potential  E  p  and corrosion potential  E  corr  for 304L and 304LL steels. Figure 4.  Percentage of obtained type N curves [data from Ref. (8)] as afunction of corrosion potential  E  corr  standard deviation. Figure 5.  Corrosion potential  E  corr  variation with immersion time in 3.5%NaCl solution for 304L steel with S1 surface finish. Figure 2.  Pitting potential  E  p  and corrosion potential  E  corr  as a functionof the surface finish for 304LL steel.  it starts to increase until a stabilized value is reached. 304Lsteel behaves in a similar manner.The S4 and S5 surface finishes display different behav-iors, which can be summarized as follows:S4 surface finish, steel 304L:  E  corr  decreases continu-ously with time until a stabilized value is reached (Fig. 7);S4 surface finish, steel 304LL:  E  corr  initially oscillatesand then decreases until a stabilized value is reached(Fig. 8);S5 surface finish, steel 304L:  E  corr  increases fast,reaches a maximum and then slowly decreases (Fig. 9);S5 surface finish, steel 304LL: its behavior, shown inFig. 10, is similar to that of steel 304LL with S4 surfacefinish, shown in Fig. 8. 4. Discussion 4.1. Correlation between E  p  and E  corr The linear correlation observed between  E  p  and  E  corr (see Fig. 3) suggests that both potentials are affected in asimilar way by the quality of the passive film formed on thesurface of the investigated metals by the different surfacefinishes, with the exception of the S2 finish. This meansthat for a given stainless steel in addition to  E  p  the  E  corr 80Alonso-Falleiros & Wolynec  Materials Research Figure 6.  Corrosion potential  E  corr  variation with immersion time in 3.5%NaCl solution for 304LL steel with S3 surface finish. Figure 7.  Corrosion potential  E  corr  variation with immersion time in 3.5%NaCl solution for 304L steel with S4 surface finish. Figure 8.  Corrosion potential  E  corr  variation with immersion time in 3.5%NaCl solution for 304LL steel with S4 surface finish. Figure 9.  Corrosion potential  E  corr  variation with immersion time in 3.5%NaCl solution for 304L steel with S5 surface finish.
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