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Determination of susceptibility to intergranularcorrosion and electrochemical reactivationbehaviour of AISI 316L type stainless steel

G.H. Aydoğdu, M.K. Aydinol *

Metallurgical and Materials Engineering Department, Middle East Technical University, Ankara 06531, Turkey

Received 11 April 2005; accepted 9 January 2006Available online 15 March 2006

Abstract

In this study, double loop electrochemical potentiokinetic reactivation (DLEPR) test was applied to

determine the degree of sensitization in 316L type stainless steel, where obtained results were correlatedwith revealed microstructures after oxalic acid test and weight loss measurements of Streicher andHuey acid tests. Best agreement was provided with test parameters which are 1 M H2SO4 and0.005 M KSCN at 0.833 mV/s scan rate at 30 C. Specimens were classified structurally as absenceof chromium carbides – step, no single grain completely surrounded by carbides – dual and one or moregrain completely surrounded by carbides – ditch, in the as-etched structure, if the I r:I a (·100) ratioswere obtained to be between 0 and 0.2, 0.2 and 5.0 and 5.0 and higher, respectively. It was also foundthat at high KSCN concentrations, reactivation current profile skewed to higher potentials where thiswas attributed the formation of metastable pits, during the anodic scan of the test procedure. 2006 Elsevier Ltd. All rights reserved.

Keywords: Intergranular corrosion; DLEPR test; AISI 316L austenitic stainless steel

1. Introduction

The best known example of metallurgical effect on corrosion is intergranular corrosionwhich is mostly observed on the use of austenitic stainless steels. Austenitic stainless steels

0010-938X/$ - see front matter 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.corsci.2006.01.003

*

Corresponding author. Tel.: + 90 312 210 25 23; fax: +90 312 210 12 67.E-mail address: [email protected] (M.K. Aydinol).

Corrosion Science 48 (2006) 3565–3583

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(containing 18% chromium–8% nickel) are widely used in steam generating plants as pip-ing and superheating tube materials due to their good mechanical properties and corrosionresistance at elevated temperatures. However, when austenitic stainless steels have under-gone a treatment like welding in the temperature range between 500 and 800 C, there is a

breaking corrosion resistance intergranularly as a result of segregation of chromium car-bides. High concentration of chromium in M23C6 particles decreases locally the chromiumcontent in the region that is adjacent to these chromium rich precipitates. Since chromiumdiffuses much more slowly than carbon, there is not enough time for chromium to diffuseto the carbide from all over the grains, so, in the region that is near grain boundaries, chro-mium content lowers to below 13%, which is a critical value for required stainless corro-sion behaviour. However, at prolonged treatments, chromium diffusion from the bulk of the grain increases the concentration above the critical limit and heals the boundaries [1].

In addition to M23C6 carbide, the exposure of AISI 316L type austenitic stainless steelto elevated temperatures for long periods of time can result in formation of various otherphases (sigma, chi, laves phases) [2–6]. The formation of the intermetallic phases, which isdelayed due to the slower diffusion of substitutional elements required for their nucleationand growth, results in a depletion of chromium and molybdenum in austenite matrix.Sigma phase with formula FeCr, which is more generally expanded as (FeNi)x(CrMo) y,is a severe problem due to its detrimental effect on the mechanical properties and localizedcorrosion resistance [7]. It nucleates mainly on the grain boundaries and is found in 316Ltype stainless steels approximately in 100 h at 800 C [4].

In austenitic stainless steels, corrosion resistance is provided by a very thin surface film,known as passive film that is an invisible film of oxide, formed by the metal reacting with

the ambient environment. Normally these films are free of pores, but their stability may beweakened locally. It therefore has different properties in areas where the steel surface isaltered due to grain boundary precipitates. This heterogeneous microstructure is very dan-gerous since it weakens steel without much change in the outward appearance.

Intergranular attack is accelerated by potential differences between grain and grainboundaries, that is, attack is determined by availability of anodic sites at grain boundaries.Therefore, making it anodic passivates the specimen. At that time, the chromium depletedalloy sets up passive–active cell of appreciable potential difference, the grains (exhibit pas-sive behaviour) constituting large cathodic areas relative to small anode areas at grainboundaries (exhibit active behaviour). During decreasing the potential, the protective pas-

sive film over chromium-depleted areas is more easily dissolved than that over undepleted(non-sensitized) surfaces. The electrochemical potentiokinetic reactivation (EPR) test isbased on the assumption that only sensitized grain boundaries become active, while grainbodies are unsensitized.

To provide a rapid, quantitative and non-destructive test method, lead many research-ers to develop electrochemical potentiokinetic reactivation tests. The history and review of EPR method were presented by Cihal and workers [8,9]. Detection of sensitization of stainless steel started with potentiostatic polarization for etching of grain boundaries byCihal and Prazak in 1956. Introduction of reactivation from transpassive or passive statewith double loop EPR technique was presented by Cihal et al. [10]. Clarke et al. on the

bases of this rapport found out firstly the single loop EPR test to quantify the sensitization[11]. This technique was also developed by Novak et al. [12]. On the other hand, doubleloop technique was presented by Desestret et al. [13], Knyazheva et al. [14], Charbonier[15], Umemura et al. [16,17] and Borella and Mignona [18] between the years 1971 and

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1980. This technique for especially detecting sensitization of 304 stainless steels was devel-oped by Majidi and Streicher [19].

In the double loop test, specimen is first polarized anodically through the active regionthen the reactivation scan in the reverse direction is carried out. When it is polarized anod-

ically at a given rate from the corrosion potential to a potential in the passive area, thispolarization leads to the formation of a passive layer on the whole surface. Then whenscanning direction is reversed and the potential is decreased at the same rate to the corro-sion potential, it leads to the breakdown of the passive film on chromium depleted areas.As can be seen in Fig. 1, two loops are generated, an anodic loop and a reactivation loop.A ratio of maximum current generated in the double loop test (I r:I a) is used as a measurefor the degree of sensitization.

In the literature, there are many studies which deal with verification of EPR testmethod, comparison of double loop and single loop test methods and improving or apply-ing test methods for different type of materials’ susceptibility to intergranular corrosion.Cihal presented a study [20], which was a continuation of an earlier study dealing withelectrochemical determination of sensitivity to intercrystalline corrosion of stainless steel,based on reactivation from the passive state. He showed that test method was verified onaustenitic chrome nickel steels with increased carbon content. The ratio of charge duringreactivation gave optimum quantitative criterion of tendency of steels to intercrystallinecorrosion. Discrepancies between the standard test (ASTM A-262 Practice E) and electro-chemical reactivation test method was shown by Novak et al. [12]. They observed that thereactivation method detects both continuous and local chromium depleted region in thesteel structure, however acid test exposes only continuous depletion zones leading to inter-

granular corrosion.The original work by Majidi and Streicher [19], which proposes the double loopmethod, compares the results of this new method and the single loop and acid test andconcludes that; the agreement between measurement made with double loop and singleloop EPR test was good and gave a quantitative measure of sensitization. It is alsoconcluded that the reproducibility of the double loop test is excellent when optimumconditions are maintained. The optimum conditions were determined by examining

Fig. 1. Diagram for the procedures of double loop EPR test method.

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parameters such as surface finish, scan rate, temperature and potassium thiocyanate con-centration which is used as an activator. They have determined the optimum DLEPR testconditions to be electrolyte of composition 0.5 M sulphuric acid and 0.01 M potassiumthiocyanate and a scan rate of 1.667 mV/s. Some specifics are such that there was an

increase not only in intergranular corrosion but also in general corrosion when lower scanrates are used. Another result was the increased I r:I a ratio as the amount of potassiumthiocyanate concentration increases because there is again general and intergranular cor-rosion at the entire surface of specimen due to the activator property of potassium thio-cyanate. However, above 0.03 M potassium thiocyanate there was no further increase inI r:I a ratio.

The area under the large loop generated in the curve of potential versus current is pro-portional to the electric charge, Q that depends on surface area and grain size and is nor-malized by total grain boundary area. This normalization is called P a that can be selectedas a tolerable level of sensitization for a given application. Majidi and Streicher also stud-ied the effects of some parameters on P a values in the single loop method for 304 and 304Ltype stainless steels. They proposed that P a value increases when decreasing scan rate,increasing roughness from 1 to 23 lm and temperature of test solution [21]. Similarlyfor 304 type stainless steels, degree of sensitization was evaluated with single loop methodby Jargelius et al. [22]. They argued that EPR test results are strongly dependent on thetesting temperature. Increasing temperature increases P a value.

In the literature, there were also some studies on the applicability of DLEPR method onhigh nickel alloys like Inconel 600, since this alloy also suffers from intergranular corro-sion. Influence of some test parameters on EPR response were investigated in order to

assess the optimum conditions which were determined to be 0.1 M sulphuric acidand 0.001 M potassium thiocyanate for sensitivity of EPR test method by Maday andMignone [23]. They discovered that at too low sulphuric acid concentration chromiumdepleted regions were not detected, while too high acid concentration caused other typesof attack. On the other hand, the optimal modified DLEPR test condition for alloy 600was obtained in 0.01 M sulphuric acid and 10 ppm potassium thiocyanate at 25 C andat 0.5 mV/s scan rate by Ahn et al. [24]. They observed that standard test conditions causepitting and general corrosion in addition to intergranular corrosion. On the other hand,the effect of potassium thiocyanate addition and its concentration on the reactivationbehaviour at single loop EPR test method of Alloy 600 in sulphuric acid solution were

investigated by Tsai and Wu [25]. They discovered that at high potassium thiocyanate con-centrations, passivation is enhanced. Tsai, Wu and Cheng also proposed that for sensitizedAlloy 600, three anodic peaks appear in the reactivation loops. While higher anodic poten-tial correspond to pitting corrosion and matrix corrosion at lower potential. The middlepotential range of +60 to 10 mV SCE was associated with grain boundary corrosion [26].

Similarly, the sensitization to intergranular corrosion of AISI 316 type stainless steelwas evaluated quantitatively both by microscopy and by electrochemical tests. The confor-mity of EPR test methods (single and double loop) and Strauss test on 18Cr–12Ni–2.5Moaustenitic stainless steel was examined by Zahumensky and Tuleja [27]. An excellent agree-ment was observed between the results of these test methods. They also found that anneal-

ing at 650 C for 100 h led to the highest sensitization among experimental states. Inanother study by Matula et al. [28], the degree of sensitization of AISI 316L type stainlesssteel to intergranular corrosion was determined by means of electrolytic etching in oxalicacid and EPR test method followed by metallographic inspection. Also the kinetics of


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precipitation of second phases were studied by means of quantitative metallography andfirst M23C6 carbides at grain boundary were detected. Chromium depletion were quanti-tatively evaluated by analytical electron microscope. They concluded that chromiumdepleted zones increases with aging time.

In the present work, optimum test parameters of DLEPR method were investigated forevaluation of susceptibility to intergranular corrosion in 316L type steels. These parame-ters are scan rate, potassium thiocyanate and sulphuric acid concentration in the test solu-tion. Nitric acid and ferric sulphate–sulphuric acid tests were also applied to specimenswhich were classified as step, dual and ditch structure by means of oxalic acid tests. Resultsof DLEPR and acid tests were correlated and finally reproducibility of convenient testparameters of DLEPR test method was controlled.

2. Experimental procedure

The chemical analysis of 316L type stainless steel used in this study was given in Table 1.Different heat treatment procedures were applied in order to simulate different degrees of sensitization, details were given in Table 2.

All specimens were dipped into a solution of 100 g of oxalic acid crystals dissolved in900 ml of deionized water. The specimens were made anode in the stainless steel beaker,which was made the cathode. The specimens were etched at 1 A/cm2 for 90 s accordingto ASTM A-262 Practice A. After applying the oxalic acid test, etched surfaces were rinsedwith deionized water and alcohol and then dried. The microstructural characterization wasmade by scanning electron microscopy (SEM) using JEOL JSM-6400 Electron Micro-

scope, and the etched structures are classified as step (absence of chromium carbides), dual(no single grain completely surrounded by carbides), ditch (one or more grain completelysurrounded by carbides).

Nitric acid and ferric sulphate–sulphuric acid tests were also conducted on all heat-trea-ted samples. After heat treatments, surface was ground by 120 grit emery paper to removeoxide scale which should be done with care. In ferric sulphate–sulphuric acid test ASTMA-262 Practice B, 236 ml of sulphuric acid is added slowly to 400 ml deionized water inorder to prevent boiling by heat evolution so that the concentration of the solution ismaintained. Then, 25 g ferric sulphate is added to sulphuric acid solution. The specimenswere not immersed with the cradle in the Erlenmeyer flask, unless the ferric sulphate was

completely dissolved in the solution. During boiling period of 120 h, the colour of solutionhas been controlled and when it changed to dark green, ferric sulphate inhibitor wasadded. In nitric acid test ASTM A-262 Practice C, a fresh 65% nitric acid was boiledand specimens were kept at this condition for five 48 h periods.

For acid tests, specimens were weighed with 0.00001 g sensitivity analytical balancebefore and after these experiments. The corrosion rate was calculated as the loss in weightas inch per month (ipm) according to ASTM A-262 as follows:

Table 1Chemical composition of 316L type stainless steel (wt.%)

C Cr Mo Ni Si Mn P S Fe

0.021 16.82 2.44 11.5 0.406 1.50 0.0338 0.0478 66.19


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ipm ¼ 278 W

A t d ð1Þ

where t is the time of exposure in hours, A is the total surface area in cm2, W is the weightloss in grams and d is the density, where for chromium–nickel–molybdenum stainless steelsit is taken as 8 g/cm3.

In the DLEPR test, first a hole of 2.5 mm diameter was drilled on one side of the 20 mmlong cylindrical specimens before the heat treatment procedure. Then the regarding heat

treatment was applied to specimens after which a 3 mm diameter thread was opened, sothat the contact between the specimen and current transfer rod is clear. Finally, all surfaceof specimen was ground by 400 up to 1200 grit emery paper. The finer finish is used for thistest to enhance the quality of photomicrographs, and also specimen was polished 1 lm

Table 2Specimen nomenclature and applied heat treatments

Name of the specimen Heat treatment time and temperature

NS At 1050 C 2.4 ks (40 min) + water quench

S-160 NS + at 650 C 576 ks (160 h) + water quenchS-233 NS + at 650 C 838.8 ks (233 h) + water quenchS-285 NS + at 650 C 1026 ks (285 h) + water quenchS-336 NS + at 650 C 1209.6 ks (336 h) + water quenchS-406 NS + at 650 C 1461.6 ks (406 h) + water quench

Table 3Variation of DLEPR test parameters and their representative experiment codes

Experiment code Test parameters

H2SO4 (M) KSCN (M) Scan rate (mV/s)

1 0.5 0.005 0.2772 0.5 0.005 0.8333 0.5 0.005 1.6674 0.5 0.005 2.55 0.5 0.01 0.2776 0.5 0.01 0.8337 0.5 0.01 1.6678 0.5 0.01 2.59 0.5 0.02 0.833

10 0.5 0.02 1.66711 1 0.005 0.277

12 1 0.005 0.83313 1 0.005 1.66714 1 0.005 2.515 1 0.01 0.27716 1 0.01 0.83317 1 0.01 1.66718 1 0.01 2.519 1 0.02 0.83320 1 0.02 1.66721 1.5 0.005 1.66722 1.5 0.01 1.66723 1.5 0.02 1.667


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alumina paste for shining appearance. During cutting, grinding and polishing operationswork piece was cooled with water to minimize temperature increase. The DLEPR testsolution is prepared freshly under a ventilated hood with stirring and used not more thanfive times due to possible breakdown of solution purity. The solution temperature is held

constant at 30 ± 1 C with the use of water bath where its temperature is controlled bythermostat control heater. The variety of test parameters can be seen in Table 3.

The electrochemical polarization cell design in this study is due to ASTM G-5 standard.In this design, the cylindrical working electrode is centrally located between the two count-erelectrodes which are placed at the sides of the cell for better current distribution andmade of materials that are inert to test solution even under strong anodic polarization.The specimen, two counterelectrodes and a calomel reference electrode, which is posi-tioned in a salt bridge, are connected to Solartron 1480 Multi Channel potentiostat.The potential of the working electrode is measured through the luggin probe, which is flex-ibly mounted to the cell and probe tip was placed near the specimen surface to minimizeIR-drop. The multistat is controlled by Corrware software, which enables the test vari-ables to be set and the results to be implemented. The procedure is composed of two steps.Firstly, the specimen was subjected to open circuit conditions for 300 s so that E corr devel-ops. Then voltage is scanned anodically from E corr to +0.3 V versus SCE with the regard-ing scan rate, after which it is reversed back to E corr.

3. Results and discussion

The resulting microstructures after the oxalic acid test were given in Fig. 2, along with

their classification according to ASTM A-262. As can be seen, the NS specimen exhibitsthe step structure, whereas S-160, S-233 and S-285 exhibit the dual structure. Althoughthe number of completely encircled grains in S-406 is more than in S-336, both are classi-fied as the ditch structure.

Results of nitric acid test (Practice C) and ferric sulphate–sulphuric acid (Practice B)test methods were given in Fig. 3. It is seen that both methods gave similar results, wherecorrosion rate initially increases with aging time, afterwards however, it is slowed downand even a slight decrease is seen. This is believed to be due to chromium re-enrichmentof the grain boundaries because of the availability of time for chromium to diffuse fromthe grain to the boundary.

In any of the combinations of the test parameters, imperceptible reactivation behaviourwas obtained for the non-sensitized specimen, which clearly depicts the state of the struc-ture. However all of the sensitized specimens, of different degree, showed a clearly recog-nizable reactivation behaviour, as it is seen from the polarization curves given in Fig. 4.

In general, for all specimens, potassium thiocyanate is more effective than sulphuricacid to increase the passivation potential and current almost irrespective of the scan rateused. Moreover, at the same test conditions, all specimens gave very similar activationbehaviour, which is desired, so that I a can be used as a reference state for the reactivationbehaviour. The reactivation current itself, however, showed a quite complex behaviourdepending on the concentrations of potassium thiocyanate and sulphuric acid, and the

scan rate. Therefore in order to understand the effect of these parameters, univariateanalysis of variance was performed on the I r:I a (·100) values to obtain a general linearmodel (GLM). The GLM univariate procedure, which was implemented in many popularstatistical analysis software, provides regression analysis and analysis of variance for one


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dependent variable by one or more factors and/or variables. Using this general linearmodel procedure, one can test null hypothesis about the effects of other variables on themeans of various groupings of a single dependent variable. Therefore one can investigate

interactions between factors as well as the effects of individual factors, some of which maybe random. In short, univariate analysis provides error estimates for each factor, and itseffect on the dependent variable, see Fig. 5, where the analysis for S-233 and S-406 spec-imens were given. Furthermore the test of null hypothesis gives the importance of the fac-

Fig. 2. SEM micrographs of specimens after the oxalic acid etch. (a) NS – step, (b) S-160 – dual, (c) S-233 – dual,(d) S-285 – dual, (e) S-336 – ditch and (f) S-406 – ditch.


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tors’ effect. If the significance of the test is lower than 0.05 then it should be understoodthat the effect of the factor is very significant. In Table 4, the significance of the GLM anal-ysis were given for the dependent variables I r:I a for all of the specimens. In Fig. 5(a), forexample, after GLM analysis we have obtained quite horizontal graphs, considering therange of error bars on the data points, for both specimens S-233 and S-406. This means

that, going from 0.5 M to 1 M and then to 1.5 M of sulphuric acid concentration didnot affect the I r:I a (·100) ratio significantly, where we have obtained 0.413 and 0.568 asGLM significance values for S-233 and S-406 specimens respectively, as given in Table4. These numbers are very much larger than 0.05, meaning that the effect of sulphuric acid

Fig. 3. Weight loss acid test results according to ASTM A-262, ipm for all specimens.

Fig. 4. Polarization curves for the NS, S-285 and S-406 specimens. Test conditions were 0.005 M KSCN + 1 MH2SO4 and 1.667 mV/s scan rate.


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is insignificant. In Fig. 5(b), however, a clear dependence of I r:I a (·100) ratio on potassiumthiocyanate concentration was seen, as it is also seen in Table 4 from the GLM significancevalues, which are 0.009 and 0.001 for S-233 and S-406 specimens respectively.

The analysis of Table 4, Fig. 5 and such figures of all of the specimens for the dependentvariable I r:I a, resulted in the following conclusions to be made. The sulphuric acid concen-tration has a weak effect on I r:I a, regardless of the state of the specimen (dual or ditch),and randomly either increases or decreases the ratio. The potassium thiocyanate concen-

(a)

(b)

(c)

Fig. 5. Results of GLM analysis. Factors variables are (a) H2SO4 molarity, (b) KSCN molarity and (c) scan rate(mV/s), and the dependent variable is I r:I a (·100). The effect of these factor variables, including the error bars, onI r:I a (·100) was given for S-233 and S-406 specimens.


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tration, however, has a strong effect on the dependent variable, such that increased potas-sium thiocyanate always decreases the ratio. Scan rate also affects the ratio considerably,such that increasing scan rate generally decreases the I r:I a ratio. If the GLM analysis weremade to see the effects of factor variables on I a and I r separately, following observationswere made. Other factors being constant and regardless of the state of the specimen, theincreased concentration of sulphuric acid causes an increase on both I a and I r, as canbe understood from its weak effect on the ratio of I r:I a. Moreover, going from dual to ditchstructure, it was observed that the absolute values of I a did not change considerably for therespective concentrations of sulphuric acid, whereas, there was a slight increase in theabsolute values of I r.

The effect of scan rate on I a, other factors being constant, was quite low for all states of the specimen, but a slight decrease can be noticed. Its effect on I r, on the other hand, isvery pronounced and as scan rate increases, I r drops considerably. Similarly, the absolutevalues of I a did not show much dependence on the state of the specimen, but for I r, therewas again a slight increase as going from dual to ditch structure. In addition, it wasobserved that, the reactivation curve expanded to active potentials with lower scan rates.This can be the sign of an increase in general corrosion rather than intergranular corro-sion. The low I r values at high scan rates, is most probably because of the insufficient time,where the passive film breakdown cannot occur effectively during reactivation scan. There-fore, I a being almost an invariant and strong dependence of I r on scan rate, it is very prob-

able to come to wrong conclusions about the state of the steel. In Fig. 6, the polarizationcurves of S-233 and S-406 were given, in which the dual structure was appeared to beexposed to more corrosion attack although its grain boundaries are more resistant to inter-granular corrosion than the ditch structure.

The effect of potassium thiocyanate concentration especially on I r was found to besomehow different from the other factors. Its effect on I a, others being constant, was suchthat I a increases considerably as potassium thiocyanate molarity increases. This increasewas observed for all specimens and the absolute values at respective potassium thiocyanateconcentrations were similar. Its effect on I r, however, was different. It was observed that athigh concentrations of potassium thiocyanate I r drops. More important than the drop

itself was the change in the reactivation profile. In Fig. 7, reactivation profiles for the dualand ditch structures were given depending on the potassium thiocyanate concentration. Itcan be seen that as potassium thiocyanate increases there is a drop in the I r, but also theprofile became skewed to higher potentials.

Table 4GLM significance values for the dependent variable I r:I a (·100) ratio for all specimens

Name of specimen Factor variables

H2SO4 KSCN Scan rate

S-160 0.873 0.024 0.220S-233 0.413 0.009 0.003S-285 0.035 0.096 0.001S-336 0.149 0.000 0.019S-406 0.568 0.001 0.010

Significance value less than 0.05 for any factor variable means that I r:I a (·100) ratio depends strongly on thatfactor variable in that specimen state.


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This is very prominent especially in the ditch structure at higher scan rates. The reasonfor this effect may be explained by the observation of a similar effect that was made in Inc-

onel 600 alloy [25,26]. In that study, the reactivation curve having two distinguishablepeaks were deconvoluted to several reactivation curves. Wu et al. [25,26] arrived to theconclusion, by comparing the microstructure of the alloy that showed the two peaksand the one that not, that the peaks were due to different types of corrosion occurringin the alloy. The deconvoluted curve appearing at higher potentials were attributed tothe pitting type of corrosion occurred in the alloy.

In this regard, the skewed reactivation profile we obtained, can be because of the com-bined behaviour of two corrosion processes taking place simultaneously, where the onetaking place at higher potentials dominating over the other one. Considering the conclu-sion of Wu et al. [25,26], we have investigated the microstructure of the S-406 specimen,

after it has been exposed to DLEPR test with different potassium thiocyanate concentra-tions. The micrographs are given in Fig. 8. It can be seen that, there is definitely a differentactivity taking place at the surface of the specimen, which is not rather the intergranularcorrosion. However this activity could not clearly be attributed to pitting type of corrosiontoo. As can be seen the number of stable pits observed in low and high potassium thiocy-anate solutions is not so different, but an increased metastable pit formation in the latter.The formation of such metastable pits can be either because of the microstructural featuresof the alloy, the repassivation mechanism or the effect of remaining surface state after theanodic scan, where the last two seems to be more likely. In order to understand this phe-nomenon, the microstructure of S-406 specimen just after the anodic scan (without reac-

tivation) was examined. The micrographs were given in Fig. 9 for the 0.005 M and 0.02 Mpotassium thiocyanate solutions. A similar microstructure was obtained compared to thecomplete scan cycle picture, Fig. 8. Therefore it is understood that the metastable pit for-mation is because of the simultaneous occurrence of passivation and film breaking, and it

Fig. 6. Polarization behaviour of S-233 and S-406 specimens under fast and slow voltage scanning conditionsrespectively. Note the lower I r value for the ditch structure compared to dual. Test solution concentration was0.5 M H2SO4 and 0.01 M KSCN.


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is enhanced in solutions containing higher amounts of potassium thiocyanate which easesfilm breaking. Such a remaining state, then causes the observed change in the reactivationprofile, and a drop in the value of I r.

In order to determine what combination of DLEPR test parameters would give the bestprediction, we correlate the results of the DLEPR and weight loss acid tests. The resultswere given in Table 5. The correlation coefficients close to one indicates that the tworesults (DLEPR and acid tests) are correlated to each other and the significance whichwas given in brackets gives how strong is the correlation, e.g. significance value less than0.05 means that there is a very strong correlation.

We have arbitrarily chosen the lower limit of correlation coefficient (for the current

ratio) to be 0.9 that is to be satisfied for all acid test, or 0.95 for one test and 0.85 forthe other acid test. The DLEPR test parameters that yielded good correlation betweenthe acid tests according to the above criteria were given with the experiment codes 8, 9,10, 12, 15 and 19. The I r:I a (·100) ratios of the above-mentioned experiments and the

Fig. 7. Reactivation profiles of specimens (a) S-233 and (b) S-406 during the DLEPR test with different KSCNconcentrations at 1 M H2SO4and 1.667 mV/s scan rate.


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ipm of Practice B and Practice C for all specimen types were given in Fig. 10. Experiments8 and 15 have been carried out with scan rates 2.5 and 0.277 mV/s, respectively. As we dis-cussed before, very high or very low scan rates may be deceptive for the determination of the state of the specimen and it is wise not use these scan rates along with any other test

Fig. 8. SEM micrographs of S-406 specimen after DLEPR test in 1 M H2SO4 solution at 1.667 mV/s scan ratewith (a, b) 0.005 M KSCN and (c, d) 0.02 M KSCN.

Fig. 9. SEM micrographs of S-406 specimen just after the anodic scan in 1 M H2SO4 solution at 1.667 mV/s scanrate with (a) 0.005 M KSCN and (b) 0.02 M KSCN.


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parameter even if they yield good correlation. Moreover the I r:I a (·100) ratios of theexperiment 8, regarding the specimen state, are close to each other, so that the resolutionof this particular experiment is low.

In the experiments 9, 10 and 19, solutions containing 0.02 M potassium thiocyanatewere used. In this condition, we should keep in mind that, during reactivation scan, notonly intergranular corrosion but also pitting type of corrosion may take place. This is seen

more obviously in the ditch structure rather than in the dual structure, so sensitizationdegree of the ditch structure should appear to be higher, where this behaviour is not incon-venient for our purposes. However, as can be seen from Fig. 10, for experiments 9 and 10,due to their low sulphuric acid content, their resolution again seemed to be low.

The final experiment in the list was given with code 12, which does not indicate any neg-ative concerns mentioned before and also it predicts the results of the Streicher acid testwith exceptional good agreement and resolution. The polarization curves of specimenstested with parameters as given in the experiment 12, which are 1 M sulphuric acid and0.005 M potassium thiocyanate and 0.833 mV/s scan rate, were given in Fig. 11. As canbe seen, there is a smooth transition as the state of the structure goes from step to dual

and to ditch.Finally to check the reproducibility of the test results, the S-233 (dual) and S-406 (ditch)

specimens were tested successively 10 more times under the conditions of the experiment12. The mean, standard deviation, standard error and 95% confidence limits for potential

Table 5Correlation and in brackets the significance values between I r:I a (·100) of DLEPR test and ipm of weight lossacid tests for Practice B and Practice C of ASTM A-262

Experiment

code

Practice B Practice C

1 0.867 (0.025) 0.946 (0.004)2 0.862 (0.027) 0.738 (0.094)3 0.637 (0.174) 0.722 (0.105)4 0.649 (0.163) 0.749 (0.087)5 0.874 (0.023) 0.802 (0.055)6 0.851 (0.032) 0.87 (0.024)7 0.698 (0.123) 0.597 (0.211)8 0.91 (0.012) 0.912 (0.011)9 0.944 (0.005) 0.908 (0.012)

10 0.989 (0) 0.869 (0.025)11 0.928 (0.008) 0.876 (0.022)12 0.98 (0.001) 0.871 (0.024)13 0.667 (0.148) 0.706 (0.117)14 0.983 (0) 0.837 (0.037)15 0.881 (0.02) 0.954 (0.003)16 0.898 (0.015) 0.865 (0.026)17 0.892 (0.017) 0.897 (0.015)18 0.703 (0.119) 0.875 (0.023)19 0.98 (0.001) 0.951 (0.004)20 0.938 (0.006) 0.851 (0.032)21 0.871 (0.024) 0.825 (0.043)22 0.814 (0.049) 0.903 (0.014)

23 0.841 (0.036) 0.968 (0.002)Correlation value close to one means that two results are correlated to each other.


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and current values were given in Table 6. It is found that, the passivation and depassiva-

tion potentials can be precisely obtained. Similarly the activation current can bereproduced within a slight error margin. However for the reactivation currents there issome variation, where its magnitude increases for the ditch structure. Nevertheless it isbelieved that, the results were reproduced within an acceptable error margin, with stan-dard errors on the reactivation current as 0.12 and 0.36 for the dual and ditch structures,respectively.

In order to classify the state of the structure according to the obtained result in theDLEPR test, we analyzed and correlated the results of all tests made and postulate that(under experiment 12 conditions), specimens giving I r:I a (·100) ratio higher than 5.0 havethe ditch structure. The upper limit for the dual structure is therefore 5.0. When the results

of all experiments for the NS and S-160 specimens were examined, the highest valueobtained for NS is in the order of 0.2 and lowest value obtained for S-160 is in the orderof 0.5. The values for the experiment 12 conditions are 0.036 and 0.782 respectively. There-fore, in order to set a clear limit from step to dual, an additional heat treatment was

Fig. 10. I r:I a (·100) ratios for the experiment codes 8, 9, 10, 12, 15 and 19, and corrosion rates according toASTM A-262 (a) Practice B and (b) Practice C.


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applied so as to yield the step structure. After the NS treatment the specimen was heattreated at 650 C for 183.6 ks. The microstructure of this specimen was characterized asstep in the oxalic acid test, see Fig. 12, and the obtained ipm values of weight loss acidtests were 0.00416 and 0.02952 for Practice B and C, respectively. The corresponding val-ues for the NS specimen were 0.0015 and 0.00166 and for the S-160 specimen they were

0.01244 and 0.15747. The I r:I a (·100) result of DLEPR test for this new specimen, underexperiment 12 conditions, was 0.3979 ± 0.065, where the statistical analysis was made oversix experiments. Then, to be on the safe side, an approximate value of 0.2 was assumed tobe the upper limit for the step structure.

Table 6Reproducibility of the DLEPR results for the S-233 and S-406 specimens with experiment 12 conditions

Specimen Mean Standard

deviation

Standard

error

95% confidence

range, lower:upperS-233

E a (V SCE) 0.146 0.009 0.003 0.153:0.140I a (A/m

2) 144.5 4.0 1.2 141.7:147.2E r (V SCE) 0.172 0.007 0.002 0.177:0.168I r (A/m

2) 3.5 0.6 0.2 3.0:3.9I r:I a (·100) 2.39 0.40 0.12 2.12:2.65

S-406

E a (V SCE) 0.154 0.007 0.002 0.158:0.149I a (A/m

2) 160.8 6.7 2.0 156.3:165.3E r (V SCE) 0.142 0.023 0.007 0.158:0.127

I r (A/m2

) 9.1 2.2 0.6 7.7:10.6I r:I a (·100) 5.67 1.19 0.36 4.87:6.47

Statistical analysis was made over 11 samples.

Fig. 11. Polarization curves of all specimens tested under experiment 12 conditions (1 M H2SO4 and 0.005 MKSCN solution at 0.833 mV/s scan rate).


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4. Conclusions

In this study, the effect of scan rate and composition on the anodic polarization and thereactivation behaviour of AISI 316L stainless steel was investigated, from which a criteriacan be obtained for the determination of susceptibility to intergranular corrosion. This cri-

teria, I r:I a ratio, is the basis of the DLEPR method. The arbitrary choice of these testparameters might be misleading. Therefore in order to devise a procedure for the correctprediction of the degree of susceptibility to intergranular corrosion in this steel, DLEPRtest parameters were systematically varied and correlated with the results of the weight lossacid tests and with the analysis of the microstructure, where finally, the following conclu-sions were drawn.

In general, I r and I a values increase similarly with the increase of sulphuric acid content,thus constituting a weak functional dependence for the I r:I a ratio.

The activation parameters weakly depends on the scan rate, but there is a strong effecton the reactivation parameters. At high scan rates, during reactivation, time is not suffi-

cient to breakdown the passive film, whereas there is plenty of time at low scan rates,which causes general corrosion to take place too. Therefore for low scan rates I r increases,so does the I r:I a ratio. Therefore the improper choice of the scan rate can yield wrongresults to be used for the prediction of susceptibility.

There is an increase in anodic current with the increased potassium thiocyanate contentin the test solution, but for the reactivation current a more complex behaviour is seen.I r always decreased for all specimen types when potassium thiocyanate is at 0.02 M con-centration compared to lower concentrations. Moreover, reactivation current profilechanges with potassium thiocyanate in such a way that it becomes skewed to higher poten-tials, where it is very obviously seen in the ditch structure. It is believed that this behaviour

is due to some surface activity taking place over the remaining microstructure of the ano-dic scan, resembling metastable pits.

DLEPR test presents quantitative results. In the evaluation of sensitization in 316Ltype steel, in terms of I r:I a the best agreement with the weight loss acid tests were obtained

Fig. 12. SEM micrograph of the specimen, after the oxalic acid etch, heat treated at 650 C for 183.6 ks (51 h),revealing the step structure.


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with the following test parameters, 1 M sulphuric acid and 0.005 M potassium thiocyanatesolution at 0.833 mV/s scan rate and with 30 C solution temperature. Increasing thepotassium thiocyanate concentration, generally, still correlates well with the acid testresults, but the resolution decreases slightly.

Using the proposed test parameters, 316L type steels are classified as ditch, dual andstep when they yield I r:I a (·100) higher than 5.0, between 5.0 and 0.2, and less than 0.2,respectively in the DLEPR test. No tolerance was allowed for step to dual limit due tosafety reasons but for the dual to ditch limit, there can be a ±1.0 deviation.

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