s15 Investigation of Pitting Potencial of Carbon Steel Using Experimental Design Methods

7/29/2019 s15 Investigation of Pitting Potencial of Carbon Steel Using Experimental Design Methods

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Investigation of pitting potential of carbonsteel using experimental designmethod

T he dependence of the pitting potential Ep

of 1018 carbon steel on chloride concentration,M.ERGUNpH and the temperature of the solution was studied by the potentiody namic method in

L. AKCAY accordance with a statistical experimental design. T he parameters of t he empirical pittingpotential model dete rmined on the basis of Box W ilson ex perimental design method were

evaluated using the experimental data. Comparison of the predicted values from the modelwith t he observed values showed that the model is a good t. From t he model equation the

most noble pitting potential value of -225 mV( SCE) was obtained when the Cl ionconcentration, temperature and pH of t he solution were 205 ppm, 25C, and 64, respect-ively. T he BoxW ilson experimental design technique was proved to be applicable in model-ling the pitting potential of carbon steel. BCJ /1798

T he authors are in t he Faculty of Engineering and A rchitecture, Gazi University, Chemical

Engin eering Department, 06570 Maltepe, A nka ra, T urkey ( [email protected]) .

Manuscript received 12 N ovember 1999; accepted 24 J uly 2 002.

2002 IoM Communications Lt d. Published by Maney for the Institute of Materials, Minerals

and Mining.

ium2 0 relating to the various environmental variables haveINTRODUCTIONbeen obtained by the present authors and also in the studyMany metals and their alloys are susceptible to pittingof Matamala.1 8 In these studies, experiments were carriedcorrosion in aqueous solutions containing aggressive ions,out conventionally, i.e., the dependency ofE

pon one of theparticularly Cl , Br , and I . The breakdown potential

variables was investigated for xed values of the remainingfor pitting Ep

characterises the resistance of metals tovariables.pitting corrosion and can, therefore, be considered to be a

An experimental design technique2 1 ,2 2 may be used formeasure of the susceptibility of diVerent metals and alloysthe empirical study of the relationship between a measuredto pitting corrosion in aggressive environments. Manyparameter on the one hand and a number of operatingpublications have appeared in the literature on the eVectsvariables on the other. This method is used to nd outof factors such as: the composition, temperature, and pH

how a particular parameter is aVected by a given set ofof the corrosive medium; the composition, heat treatment,operating conditions over some specied region of interestdegree of cold work, and structure of the metal or alloy;and to determine the values of operating conditions whichand the structure of the oxide lm on passivated metals onwill yield a maximum for t he specic parameter as a resultthe mechanism, rate, and other parameters of pittingof optimisation. The major advantage of applying experi-corrosion.1 1 0mental design is to reduce the number of experiments thatThe chloride ion is generally regarded as the mosthave to be carried out to obtain as much information asaggressive pitting agent and it has been the most thoroughlypossible in the most eVective way.studied ion because of its wide distribution in nature. It

The main objective of this paper is, therefore, to relatehas been found that the breakdown potential for pitting isthe eVects of some environmental conditions, namely thea logarithmic function of the chloride ion concentration.temperature, chloride ion concentration, and pH of theIncreasing chloride ion concentrations shift the pittingsolution, on E

pvia the BoxWilson experimental designpotential to more active (i.e. more negative) values.

technique which has proved very useful in other elds,Some publications have examined the inuence of

especially those relating to chemical and microorganismtemperature on pitting.7 ,8 ,1 1 1 4

In general, Ep decreases reactions.2 2 2 5 The conditions which yield the most noblewith increase of temperature for all types of stainless steel,value of E

pfor the selected region of interest and thealthough the particular dependence on temperaturedepends

response surfaces for the predicted model are presented.on the grade of stainless steel.There is no general agreement concerning the eVect of

pH on the pitting potential. Some reports indicate that theMATERIALANDMETHODvalue ofE

pvalue is constant over a large range of pH. 7 ,1 5 1 7

ExperimentalmethodIt has also been reported that pitting of austenitic stainlessSpecimens were prepared from carbon steel rods of thesteels is strongly inhibited at pH values above 9 and thatfollowing composition (wt-%): Fe018C003Si050Mna constant value for E

pis maintained in the acid range.7 A

004S005Mo015Ni. Copper wires were attached to therecent study of AISI 316L stainless steel1 8 reported aback of cylindrical specimens cut from the rods. After thevariation in E

pof only 30 mV when the pH value was

wires had been enclosed in glass sleeves, the specimenschanged from 36 to 11.were mounted in an epoxy resin, leaving an area ofSeveral studies exist concerning the independent eVects

~10 cm2 exposed to the solution.of initial pH, temperature or chloride ion concentration. InExperiments were performed in a three compartmentindustrial applications, there are various media that create

Pyrex cell with a platinum counter electrode and avery aggressive environments so that many alloys maysaturatedcalomel electrode( SCE) as the reference electrode.suVer localised corrosion by pitting. Once an alloy hasAll potentials are reported with respect to SCE. Prior tobeen selected for such an application, material damage byeach electrochemical run, the electrodes were ground withlocalised corrosion is very diYcult to control. However,3/0 grade emery paper and then polished with alumina,one possible way of controlling such damage is to knowdegreased, rinsed thoroughly with water and placed in thethe variation of E

pfor a particular alloy or metal as a

test solutions for 1 h. Solutions were prepared with distilledfunction of the process variables. Some mathematicalmodels for E

pof 16 wt-%Cr stainless steel1 9 and alumin- water and reagent grade chemicals and were open to

DOI 10.1179/000705902225006598 British Corrosion Journal 2002 Vol. 37 No. 3 235


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236 Ergun and Akcay Pitting potential of carbon steel using experimental design method

the atmosphere and magnetically stirred throughout the experiment (Table 1). The coded values for the independentvariables and the corresponding real values are givenexperiments.

Anodic potentiostatic polarisation measurements were in Table 2.undertaken using an EG & G Model 362 scanningpotentiostat. A Servagor 120 XY recorder was used for RESULTSANDDISCUSSIONthe simultaneous measurement of potential and current.

The result at each point based on the experimental designAfter the period at open circuit, the working electrodes

is given in Table 3. The BoxWilson experimental designswere anodically polarised with a scan rate of 1 mV s 1 .

are a general series of experiments that have been developedThe potential at which the anodic current increased

to serve as an eYcient basis for deriving the mathematicalmarkedly was taken as the pitting potential. Agitation wasmodel of a physical process. Their usefulness is enhanced

provided by a heating magnetic stirrer and throughout thein the study of industrial applications because most physical

experiment the temperature was controlled with a thermis-situations can usually be approximated by a quadratic

tor probe in conjunction with the magnetic stirrer. Allfunction over a reasonable range of factors. Therefore the

experiments were repeated at least twice to verify theirmodel of regression tted is

reproducibility.Y=b

o+

i

bix

i+

i j

bijx

ix

j. . . . . . (5)

Experimentaldesignwhere Y is the predicted response, subscripts i and j vary

The BoxWilson experimental design was used in thefrom 1 to the number of variables, b

ois the intercept term,

optimisation of pitting potential. Temperature X1

(C), pHb

ivalues are linear coeYcients, and b

ijvalues are quadratic

X2

, and chloride ion concentration X3

(ppm) were chosencoeYcients. This was a square regression model in terms

as independent factors in the experimental design. Pittingof coded values. Parameters of this equation were evaluated

potential Ep (mV) was the dependent output variable. For from the experimental results of specic experimentsconvenience the independent variables in the model aredesigned to determine their value (Table 3) with the

utilised in their coded form. The variables Xi

were codedSPSS/PC (Statistical Package for the Social Sciences, SPSS;

as xi

according to the equationChicago, IL, USA). The resultant functional relationship

xi= (X

i-X0

i)/DX

i. . . . . . . . . (1 ) in terms of coded values for predicting pitting potential

values of carbon steel waswhere x

iis the coded value of the variable X

i, X

iois the

value of Xi

at the centre point of the investigated area, and Y=-583 7-452x1-19 9x

2-104x

3+342x2

1DX

iis the step size. In this study, more specically

+152x22+55x2

3-590x

1x

2+55x

1x

3+45x

2x

3x

1=(X

1-45 )/12 . . . . . . . . . (2 )

. . . . . . . (6)x

2=(X

2-45)/12 . . . . . . . . . (3 )

This equation includes all the terms regardless of theirx

3=(X

3-550)/260 . . . . . . . . . (4 ) signicance. It can be seen by the high value of regression

coeYcient, R=0994, that the model is a good t. ToFor tting a second order model, the BoxWilson experi-determine the signicance of each coeYcient, a statistical

mental plan with six experiments at star points and withanalysis was conducted. The decision about the signicance

six replicates at the centre point, with a total number of 20is based on the parameter value (P value). The value of a

experiments, was employed where a is a coded value forrepresents the level of signicance. If the parameter value

the star point. It is taken as equal to k1 / 2 , where k representsis less than the preassigned level a, then the results are

the number of variables.2 6 So, a is equal to 1732 for thissatisfactorily signicant at level a. The most commonly

experimental design. The data at the (0,0,0) point areused level of signicance is 005. When the signicance

needed to obtain a measure of the error involved in thelevel is set at 005; a P value under 005 would be signicant.Since b

3 3and b

1 3values were found to be statistically

insignicant for a a value of 005, these variables wereTable 1 Experimental plan with coded values fordropped from the regression equation. The coeYcients ofindependent variables of temperature x1 ,the new estimated regression equation, which was obtainedpH x2 , and chloride ion concentration x3

by the removal of the insignicant coeY

cients, are given in(a=1732 in star point)* Table 4 together with the P values. It can be seen from thistable that all of the remaining coeYcients are statisticallyExperiment x1 x2 x3signicant because of having lower P values than a when

1 +1 +1 +1 a is equal to 005. So the identied statistical model dening2 +1 +1 -1 E

pfor carbon steel is given below (with R of 0991)

3 +1 -1 +1Y=-578 9-452x

1-19 9x

2-79x

3+335x2

14 +1 -1 -15 -1 +1 +1

+145x22-590x

1x

2+88x

2x

3. . . . . (7)

6 -1 +1 -17 -1 -1 +18 -1 -1 -1

Table 2 Real and coded values of independent9 +a 0 010 -a 0 0 variables in experimental plan11 0 +a 0

12 0 -a 0 Real values13 0 0 +a

14 0 0 -a Coded X1 X2 X315 0 0 0 values (temperature), C (pH) (Cl concentration), ppm16 0 0 0

17 0 0 0 -1732 25 25 10018 0 0 0 -1 33 33 29019 0 0 0 0 45 45 55020 0 0 0 1 57 57 810

1732 65 65 1000* Corresponding real values X1 , X2 , and X3 are given in Table 2.

British Corrosion Journal 2002 Vol. 37 No. 3


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Ergun and Akcay Pitting potential of carbon steel using experimental design method 237

The predicted values of Ep

are displayed in Table 3, nounced for changes in temperature at low levels. At highertemperatures, relatively smaller shifts of E

pin the negativetogether with the observed values. Comparison of these

predicted values with those determined experimentally direction were observed. According to the data for AISI316 reported by Leckie and Uhlig7 in neutral chlorideindicates that in no cases did the diVerences exceed 34%.

This equation is valid for temperatures between 25 and solutions and by Suzuki and Kitamura1 in chloridesolutions at pH 55, there is a greater temperature depen-65C, for acidic pH values ranging from 25 to 65, and the

chloride ion concentration range ( 1001000 ppm) consid- dence of Ep

for this material at temperatures below 25C,which is consistent with the situation found in this work.ered in this study.

The maximum of the pitting potential was obtained with At this pH value, the most noble value of Ep

was obtainedwith the lowest temperature used (25C).the Mathematica software using the steepest ascent tech-

nique. Equation (7) gave the maximum value of Ep

when Figure 1b represents the response surface correspondingto varying pH and temperature values for the xed valuex

1=-1732, x

2=167, x

3=-132. Using these values and

equations (2)(4 ), the pH value was calculated as 64 and of Cl concentration of 205 ppm and shows that the eVectof pH depends on the value at which the temperature wasthe temperature and chloride ion concentration were found

to be 25C and 205 ppm, respectively. The most noble xed. For higher values of the temperature, Ep

moves inthe active direction as pH increases, but with lower valuespitting potential value corresponding to these independent

variable values was -225 mV. This maximum value is the of temperature the reverse happens. There have been fewstudies devoted to the eVect of pH on E

pand also there ismaximum within the range of experimental values.

The response surfaces in Fig. 1 are based on the above no general agreement for this dependence.7 ,1 5 1 7 It wasstated that for 188 stainless steel E

pwas not aVectedmodel with one variable kept constant at the level

corresponding to the most noble Ep

value, and varying the appreciably in 01 N NaCl at 25C for pH values in therange 1 to 7. However, in the study by Matamala1 8 onother two within the experimental range. With these

response surfaces it is possible to examine the dependence AISI 316L stainless steel using spent bleach solution, it wasreported that E

pmoved by an amount equal to 26 mVof E

pon the environmental conditions selected.

In Fig. 1a, the p H wa s xed at 6 4 . An increa se in p H 1 in the active direction for acidic pH values. Thisnding is supported by the results of the present study.temperature leads to shifts of E

ptowards more negative

values. This dependence was the same for all levels of The most noble values of Ep

were observed at the lowesttemperatures and highest pH values used.chloride ion concentrations for this pH value. It was

generally reported that Ep

decrea ses with in crea se o f In Fig . 1c, where the temperature was xed at 25C, Ep

decreases as the chloride ion concentration increases. Thetemperature for all types of stainless steel, although theparticular dependence on temperature depends on the eVect of chloride ion on E

phas been widely investigated

and there is a general agreement that as the Cl grade of steel.7 ,8 ,1 1 1 4 The data obtained in this investigationsupport this contention. Changes in E

pare more pro- concentration increases, E

pchanges in the active direction,

Table 3 Experimental design and comparison of experimental and predicted pitting potential ofcarbon steel*

Coded values Real values Pitting potential Ep

Experiment x1 x2 x3 X1 , C X2 X3 , ppm Experim ental, m V Predicted, m V %Error

1 1 1 1 57 57 810 -662 -6541 122 1 1 -1 57 57 290 -654 -6559 023 1 -1 1 57 33 810 -508 -5139 114 1 -1 -1 57 33 290 -495 -4884 135 -1 1 1 33 57 810 -450 -4457 096 -1 1 -1 33 57 290 -433 -4475 347 -1 -1 1 33 33 810 -545 -5336 21

8 -1 -1 -1 33 33 290 -497 -5087 229 a 0 0 65 45 550 -548 -5561 16

10 -a 0 0 25 45 550 -410 -4001 2411 0 a 0 45 65 550 -572 -5700 0312 0 -a 0 45 25 550 -500 -5008 0113 0 0 a 45 45 1000 -582 -5925 1814 0 0 -a 45 45 100 -548 -5650 3115 0 0 0 45 45 550 -580 -5789 0116 0 0 0 45 45 550 -583 -5789 0717 0 0 0 45 45 550 -585 -5789 1018 0 0 0 45 45 550 -578 -5789 0119 0 0 0 45 45 550 -584 -5789 0820 0 0 0 45 45 550 -592 -5789 22

* X1 temperature; X2 pH; X3 Cl concentration; a=1732 in star point. %Error=[(Ep,predicted-Ep,experimental )/Ep,experimental ]100.

Table 4 Coefficients and related parameter values (P values) in regression model

b0 b1 b2 b3 b11 b22 b12 b23

Coefficient -5789 -452 -199 -79 -335 145 -590 88P value 40810 21 29510 9 210 5 00196 41710 8 00002 34510 9 00442



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238 Ergun and Akc ay Pitting potential of carbon steel using experimental design m ethod

ion concentration. From Fig. 1c it may also be seen thatfor all xed Cl ion concentrations E

ptends to be more

noble at this low temperature value as pH increases, inagreement with the results of Fig. 1b. The value of E

pis

again at its most noble (-225 mV) under the aboveconditions.

In all three of these gures it was observed that the mostnoble value of E

pwas obtained at the lowest chloride ion

concentration and temperature values and the highest pHvalue. Therefore the optimum conditions obtained fromthe model were reconrmed by examination of theresponse surfaces.

CONCLUSIONS

The BoxWilson experimental design method is applicableto modelling of the pitting potential of carbon steel. Themodel developed allows easy determination of E

pvalues

and variations in Ep

depending on changes of the systemvariables: pH; temperature; and chloride ion concentration.Knowledge of E

pmakes it possible to control material

damage caused by pitting. It would be useful to study the

validity of this type of model for other metals known to besusceptible to pitting corrosion.

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1 Response surface of pitting potential Ep

according to the24. u. gunduz and m. ergun: DOGA T urkish J . E ng. Environ. Sci.,empirical model calculated from results obtained in

2001, 25, 11.accordance with experimental plan (Table 1) as function of25. u. gunduz and k. korkmaz: Proc. 11th Conf. on Partitioningtemperature, pH, and Cl concentration

in aqueous two-phase systems, Gulf Shores, AL, USA,JuneJuly 19 99, University of Alabama.

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pindicates that the resistance to pitting handbook, 5th edn, Section 2; 1974, New York, NY,

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s15 Investigation of Pitting Potencial of Carbon Steel Using Experimental Design Methods