corrosion resistance of austenitic cr-ni … · 21 int. j. mech. eng. & rob. res. 2014 nitesh...

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Int. J. Mech. Eng. & Rob. Res. 2014 Nitesh Kumar et al., 2014

CORROSION RESISTANCE OF AUSTENITIC Cr-NiSTAINLESS STEEL IN 1 M HCl

Nitesh Kumar1*, Ajit Kumar1, Anjani Kumar Singh2 and G Das2

*Corresponding Author: Nitesh Kumar,[email protected]

The corrosion resistance of austenitic stainless steel grade 316 were performed in 1 Mhydrochloric acid solution using the electrochemical potentiostatic polarization method. Theresult showed the more active corrosion reactions in presence of chloride ion results in pittingcorrosion observed on the metal surface. The potential range of passivity was shortened andboth the pitting potential and protection potential shifted towards the active direction. Potentiostaticpolarization method was used for the pitting corrosion investigation. The electrochemical corrosionreactions exhibited both the passive and active corrosion reactions characteristics. The acidsat the intermediate concentrations show more obvious active corrosion reactions; while in theconcentrated form they were relatively passive—the passivity that was associated with theoxidizing nature of the concentrated acids.

Keywords: Pitting corrosion, AISI 316 stainless steel, Hydrochloric acids, Pit nucleation

INTRODUCTIONGrade 316 has excellent corrosion resistancewhen exposed to a range of corrosiveenvironments and media. It is usually regardedas “marine grade” stainless steel but is notresistant to warm sea water. Warm chlorideenvironments can cause pitting and crevicecorrosion. The existing demand for materialswith high functional properties, a definitegeometrical shape, a high resistance to the

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1 Materials Science and Engineering, Scholar National Institute of Foundry and Forge Technology Hatia, Ranchi, Jharkhand 834003,India.

2 Materials and Metallurgical Department, National Institute of Foundry and Forge Technology Hatia, Ranchi, Jharkhand 834003,India.

destructive effect of a aggressiveenvironments, produced in compliances witha valid ecological standards, motivates themanufacturers to a continuous improvementin the engineering process, in particular thesteelmaking and plastic forming processes.Stainless steels fulf il enough aboverequirements and characterized excellentcorrosion resistance combined with highmechanical and plastic properties. The most

Research Paper

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common stainless alloys are austenitic steels,well-known as 18/8 types and usually containbetween 16 and 18%wt. Cr, 6-8%wt. Ni and0.03-0.1%wt. C (Lamb, 2001; and Baronet al., 2006).

Stainless steels are iron alloys that havethin, transparent and durable chrome oxidepassive layers on the surface. The stability ofpassive film and the corrosion resistance ofstainless steel increase with increasingchrome content in the alloy. Although the costsof the stainless steel are higher than those withsimilar mechanical characteristics, the primaryjustification for its common usage is improvedcorrosion resistance. Stainless steels have alarge range of applications. Over 1/3 ofproduced stainless steels are used in structuralapplications for chemistry and powerengineering industries. These applicationsinclude nuclear reactor canals, heat converters,tubes that are used in oil industries, chemistryapplications, paper industries components,and as the pieces of boilers and furnaces thatare used in nuclear reactors (Kajzer et al.,2007).

Corrosion resistance of stainless steelsdepends on various metallurgical andprocessing variables (Opiela et al., 2009). Theaustenitic grade is considered to be mostresistant to industrial atmospheres includingaggressive aqueous and non-aqueous acidmedia (Surowska and Weroski, 1995).

However, as conditions become moresevere, addition of several alloying elementsis useful in promoting corrosion resistanceand hence is desirable. Chromium >12%wt.,for instance, improves the passivity of ironalloys, and molybdenum (>2%wt.) promotes

resistance to pitting corrosion (Zor et al.,2009).

The exorbitant amount spent on corrosionand protection of engineering components,structures/facilities annually has created a lotof interest in continually seeking for techniquesand methods to efficiently combat corrosion.This is of particular interest to corrosionscientists and engineers worldwide, to gainmore knowledge about the corrosionphenomenon and its control, and with the aimof finding appropriate and better utilization ofaustenitic stainless steels and other alloys(Loto, 2012).

The austenitic stainless steels comprise alarge and varied group of iron-based alloyscontaining 18% or more chromium andsufficient nickel to assume a fully austeniticmetallurgical structure. The most commongroup is referred to as “18-8” stainless steel18%Cr and 8%Ni of which Type 316 stainlesssteel belongs. These alloys are of majorimportance in the chemical process industriesand in other industrial applications. They arethe alloys of choice for many of the applicationswhich require a higher level of corrosionresistance than the type 304 stainless steel. Incomposition, type 316 consists of about 2%Mo to improve the resistance to pitting attackby strengthening the passive film and impartingimproved corrosion resistance in reducingacids.

Austenitic Stainless steels are used asconstruction materials for key corrosion-resistant equipment and due to their strengthin most of the major industries, and especiallyin chemical, petroleum, offshore drilling marineshipping, water desalination, processing, and

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power generation plants. The high corrosionresistance of this material arises from theformation of a passive layer on its surface. Thispassive film or layer is very thin, which is selfhealing in a wide variety of environments,provides their corrosion resistance. Due toenvironmental concerns, there has recentlybeen an increase in the demand for ethanolas a road vehicle fuel. There is therefore, agreater need for studies of the corrosionresistance of materials used in alcoholproduction plants, and in transportation andstorage facilities (Pardo et al., 2008).

Additionally, chromium content in the steelenhances its corrosion resistance. Thepassive film is constituted by an iron andchromium oxy-hydroxide layer and watercontaining-compounds which is formed at themetal/solution interface, and an underlying filmformed by chromium oxide. However, undercertain circumstances, the passive state maybe lost, and most stainless steel-basedequipment failures are caused by pittingcorrosion due to chloride ions (Fontana andGreene, 1967).

Hydrochloric acid remains the main sourceof the aggressive chloride ions. Pittingcorrosion is one of the most dangerous formsof localized corrosion especially for steels inchloride media (Aldahan, 1999). On the otherhand, sulphuric acid is very corrosive forstainless steels too, and constitutes one of thebasic raw materials encountered in thechemical industry. Despite the number ofpublications about stainless steel corrosionand passivation, the kinetics of the variouscomplex processes involved has beeninvestigated less extensively in concentratedsolutions of these acids. Therefore, it is

interesting both from the fundamental andpractical standpoint to study the influence ofacidity, chloride and sulphate ions on thoseprocesses. The presence of such ionsimposes the use of inhibitors to avoiddestruction of either the material surface or thepassive layer in contact with the aggressivesolution (Ait Albrimi, 2011).

Electrochemical studies of pitting corrosionhave found the existence characteristicpotentials. Stable pits form at potentials nobleto the pitting potential, Ep, and will grow atpotentials nobler than there passivation one,ER, which is lower than Ep. During upwardscanningin a potentiodynamic polarizationexperiment, a stable pit starts growing at Ep,where the current increases sharply from thepassive current level and, upon reversal of thescan direction, repassivates at ER where thecurrent drops back. It is generally consideredthat materials exhibiting higher values of Epand ER are more resistant to pitting corrosion,and potentiodynamic polarization experimentsare commonly used for this purpose.6, Ep isin many cases a function of experimentalparameters such as potential scan rate.Furthermore, so-called metastable pits initiateand grow for a period at potentials well belowthe pitting potential, which provides evidencein contradiction to the definition of the pittingpotential as being the potential above whichpits initiate. Metastable pits initiate and growfor a limited period before repassivating.Large pits can stop growing for severalreasons, but metastable pits are typicallyconsidered to be those of micron size at most,with life times on the order of seconds or less.

In the present investigation, the corrosionbehaviour of AISI 316 stainless steel exposed

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to hydrochloric acid solution has beeninvestigated with special focus on thecharacterization of the pitting susceptibility ofthe steel in these environments. The influenceof various factors such as the nature of theacidic media and its concentration, potentialcycling, potential scan rate, and the additionof Cl- ions were considered.

EXPERIMENTAL METHODAustenitic stainless steel AISI 316 plates havebeen used. Table 1 provides its nominalchemical composition. The composition ofsteels as analyzed by optical emissionspectrophotometer is given in the Table 1.

Austenitic stainless steel-SS 316 samplesin cylindrical form (20 mm dia. and 6 mm long)used for this investigation. The surface finishingprocedure recommended by ASIM wasreplaced by the polishing procedure used asstandard practice in metallographiylaboratories, i.e., wet grinding successivelywith grades 80, 120, 180, 220, 320, 400, 600and 800 silicon carbide papers and then clothpolishing with alumna. The finishing waschecked under the optical microscope at 100x.This finishing permits a clearer resolution ofthe metallic surface structure. The sampleswere immediately kept in desiccators forsubsequent corrosion experimental studies.

Electrochemical studies were carried out ina conventional three-electrode single-compartment Pyrex glass cell (Model NOVA1.18, serial no. 12800817 Pyrex cup, 300 mlvolume, with the cover model AUTOLAB one

channels/openings, software NOVA 1.8.17).Sample was used as the working electrodesby putting only one side of the specimen incontact with the electrolyte (1 cm2).

The remaining sides were covered with anon-reactive and non-conductive varnish. Theelectrical contact with the test electrode wasmade using a crocodile clip attached on asmall strip of the sample.

Potentiodynamic experiments wereperformed using each of the cylindricalspecimens in turns, in which 1 cm2 surface areaof the specimen was exposed to the testsolution at room temperature. The experimentswere performed using a polarization cell ofthree – electrode system consisting of areference electrode (saturated-AgAgCl/KCl0.197V), a Working Electrode (WE); and aCounter Electrode (CE) made of platinummesh. The experiments were conducted in 1Mole HCl solution for stainless steel sample.

The electrochemical measurements wereperformed using a computerizedpotentiostatic electrochemical setAUTOLABmodel PGSTAT 3210potentiostate. The corrosion behaviour of thestainless steel was investigated 1 mole HClacid solution, at room temperature, usingpotentiodynamic polarization technique. Testenvironment were 1 M HCl (33.6 ml) 266.4 mldistilled water.

Potentiodynamic polarization measurementswere performed at a scan rate of 0.5 mV/s,initial potential-0.25 V final potential 1.6 V,

Elements C Mn Si P Cr Ni Mo V Cu Sn Al

Wt% 0.08 0.44 0.019 0.009 16.67 9.86 2.09 0.070 0.49 0.013 0.042

Table 1: Chemical Composition of Grade 316 Austenitic Stainless Steel

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step height 0.5 mV, step time 1 mV andmeasured open circuit –300.572 mV withrespect to the corrosion potential (Ecorr) andscanning back to the starting potential whenthe samples reached a current density valueof 104 l A/cm2.

RESULTS AND DISCUSSION1 Mole Hydrochloric Acid (1 M HCl)EnvironmentThe potentiostatic polarization curves andtable of measurement results for the testspecimen recorded in 1 M HClarepresented in Figure 1 and Table 2respect ively. Figure 1 shows thepolarization corrosion curve of the 316stainless steel in 1M HCl alone. The OpenCorrosion Potential (OCP), Ecorr was–0.305 V. The specimen can be describedto be protected with this potential valuethroughout the experimental period. Thismedium serves as the control for thesubsequent experiment. A summary of theoverall result data is presented in Table 2.

Results summary for the test in 1M HClpresented in Table 1 showed that the corrosionrate is 42.514 mm/yr and the corrosionpolarization resistance (R) is 11.034. Thecorrosion current density of 0.003659 A/cm2

was low and the corrosion current is 0.003659A which was also very low. All these results dataconfirmed the corrosion of the test specimen1 M HCl to be very high.

Figure 1: Potentiodynamic PolarizationCurves 316 Austenitic Stainless Steel

in 1 M HCl

1 0.003659 0.003659 0.08779 –1.5819 11.034 –0.4073 –0.4080 –0.4130 –0.3981 42.514

Table 2: Electrochemical Parameters Estimated from the Polarization Tests in 1 M HCl

Conc.(M)

Icorr(A)

Icorr (A/Cm2)

bc (V/dec)

ba (V/dec)

Rp(Ohm)

EcorrReference

(V)

EcorrCalc(V)

Ebegin(V)

Eend(V)

Corrosionrate(mm/year)

CONCLUSIONThe electrochemical behaviour of type 316austenitic stainless steel in acidic solutionsdepends considerably on the concentration ofhydrochloric acid environment. The presenceof acid concentration produces andenhancement of metal corrosion through thepassive layer, and decreases the passivitybreakdown potential. Pit nucleation and growthinvolves a number of contributions which canbe distinguished through the analysis of currenttransients at constant potential.

The corrosion polarization behaviour for thetests performed in hydrochloric acid showedactive corrosion reactions behaviour at all theused concentration (1 M). Theless corrosionresistance of the test electrode as indicatedby the obtained results data.

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The overall corrosion resistanceperformance of this alloy in the testenvironments can be rated as fair to good.

REFERENCES1. Ait Albrimi Y (2011), “Electrochemical

Behaviour of AISI 316 Austenitic StainlessSteel in Acidic Media ContainingChloride Ions”, International Journal ofElectrochemical Science, Vol. 6, pp.4614-4627.

2. Aldahan H A (1999), J. Mater. Sci.,Vol. 34, p. 851.

3. Baron A, Simka W, Nawrat G, SzewieczekD and Krzyak A (2006), “Influence ofElectrolytic Polishing on ElectrochemicalBehaviour of Austenitic Steel”, Journal ofAchievements in Materials andManufacturing Engineering, Vol. 18,pp. 55-58.

4. Fontana M G and Greene N D (1967),Corrosion Engineering, p. 51, McGraw-Hill, New York.

5. Kajzer W, Chrzanowski W and MarciniakJ (2007), “Corrosion Resistance of Cr-Ni-Mo Steel Intended for UrologicalStents”, International Journal ofMicrostructure and Materials Properties,Vol. 2, pp. 188-201.

6. Lamb S (2001), Handbook of StainlessSteel and Nickel Alloys , CASTIPublishing INC., ASM International.

7. Loto C A (2012), “Corrosion PolarizationBehaviour of Type 316 Stainless Steel inStrong Acids and Acid Chlorides”,International Journal of ElectrochemicalScience, Vol. 7, pp. 3787-3797.

8. Opiela M, Grajcar A and Krukiewicz W(2009), “Corrosion Behaviour of Fe-Mn-Si-Al Austenitic Steel in ChlorideSolution”, Journal of Achievements inMaterials and ManufacturingEngineering, Vol. 33, No. 2, pp. 159-165.

9. Pardo A, Merino M C, Coy A E, Viejo F,Arrabal R and Matykina E (2008), Corros.Sci., Vol. 50, p. 780.

10. Surowska B and Weroski A (1995), “ColdWork Effect of Pitting Corrosion of SomeBiomaterials”, Proceedings of the 14th

International Scientific Conference“Advanced Materials and Technologies”,pp. 425-428, Gliwice Zakopane.

11. Zor S, Soncu M and Capan L (2009),“Corrosion Behaviour of G-XCrNiMoNb18-10 Austenitic StainlessSteel in Acidic Solutions”, Journal ofAlloys and Compounds , Vol. 480,pp. 885-888.

corrosion resistance of austenitic cr-ni … · 21 int. j. mech. eng. & rob. res. 2014 nitesh...

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