HIE Wear.pdf

CORROSION SCIENCE SECTION

Hydrogen-Induced Embrittlement Wearof a High-Strength, Low-Alloy Steelin an Acidic Environment

T.C. Zhang,* X.X. Jiang, and S.Z. Li**

ABSTRACT

Corrosive wear of a high-strength, low-alloy steel (HSLA) wasexamined in 0.02 mol/L sulfuric acid (H2SO4) solution atdifferent polarized potentials and loads using a pin-on-discwear device and a potentiostat. Morphologies of the weartracks were observed by scanning electron microscopy(SEM). Hydrogen content in the surface or subsurface of weartracks was determined using secondary ion mass spectrom-etry (SIMS). Results showed the increased material removalwith a negative shift of potential in the cathodically polarizedrange resulted from the synergistic effect of hydrogen-induced damage and mechanical wear from hydrogen evolu-tion on the wear surface. Increases in wear loss withpotential in the anodically polarized range resulted fromsynergism between anodic dissolution and wear.

KEY WORDS: corrosive wear; embrittlement; high-strength,low-alloy steel; hydrogen-induced damage; morphology;scanning electron microscopy; secondary ion massspectrometry

INTRODUCTION

High-strength, low-alloy steels (HSLA) play an impor-tant role in the aviation, aerospace, navigation, andoil and chemical industries. Much work has been

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Submitted for publication October 1995; in revised form, June1996.

* Department of Materials Physics, University of Science andTechnology Beijing, 100083 Beijing, China.

** Institute of Metal Research, Academia Sinica, 110015, Shenyang,China.

(1) UNS numbers are listed in Metals and Alloys in the UnifiedNumbering System, published by the Society of AutomotiveEngineers (SAE) and cosponsored by ASTM.

0010-9312/97/00005© 1997, NACE

published concerning fracture failure of HSLA steelsin aggressive media.1-4 Hydrogen-induced fracture ofHSLA steels from shearing stress has been noted.5-7

However, the corrosive wear behavior (i.e., simulta-neous damage by surface-shearing stress andchemical aggression) remains ambiguous.

The trend toward residual fuels and away fromdistillate fuels has increased the incidence of corro-sive wear in marine diesel engines from sulfuric acid(H2SO4). Severe corrosion and corrosive wear havebeen reported on cast iron cylinder linings and pistonrings in these engines. Unfortunately, corrosive wearbehavior of HSLA steel cylinder linings or pistonrings under these conditions is not clear.

Generally, under applied cathodic polarization ofan alloy in acidic media, hydrogen will be evolvedfrom the alloy surface. Belyi, et al., discussed hydro-gen separation in friction and its effect on wear.8 Ithas been reported that hydrogen evolution duringwear can accelerate damage of titanium alloys.9

Generally, hydrogen comes from decomposition oflubricating oil, cylinder oil, and the cathodic processof corrosion and corrosive wear of the steel.

The objective of the present work was tostudy the corrosive wear behavior of HSLA steel in0.02 mol/L H2SO4 solution, focusing on the effect ofpotential on wear resistance, to better understandthe effect of hydrogen evolution.

EXPERIMENTAL

The HSLA steel used (UNS H43400)(1) was meltedin a vacuum induction furnace. Chemical composi-

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FIGURE 1. Microstructure of the HSLA steel.

FIGURE 2. Schematic of steady corrosive wear tester: (1) basement,(2) electrolytic cell, (3, 6) strain gauges, (4) x-y recorder, (5) loadingrod, (7) bearing, (8) brush, (9) gear box, (10) reference electrode,(11) auxiliary electrode, (12) corrosion measurement system, (13)motor, (14) specimen, and (15) axis.

tion was (wt%): 0.32% C, 0.20% Si, 0.70% Mn,0.90% Cr, 1.0% Ni, 0.85% Mo, 0.20% V, and balanceFe. The steel was normalized at 890°C, quenchedat 870°C in water, and then tempered at 450°C.Figure 1 shows the resulting microstructure.

After heat treatment, the test sample was ma-chined into a ring specimen 38 mm (1.5 in.) indiameter and 10 mm (0.4 in.) in thickness, ground toa roughness of 0.6 µm (Ra), degreased, cleaned, anddried. The periphery of the ring was covered withsolid paraffin for isolation.

A pin-on-disc corrosive wear apparatus was usedto measure material removal and electrochemicalvariables simultaneously in 0.02 mol/L H2SO4 solu-tion (Figure 2). A polytetrafluoroethylene (PTFE)electrolytic cell was fixed on the stainless steel (SS)basement, the ring specimen was mounted on thehollow axis, and the ring specimen and brush wereconnected using a conducting wire through the hol-low axis. The rider was an alumina (Al2O3) ball 6 mm(0.24 in.) in diameter fixed onto the bottom of theloading rod. A saturated calomel electrode (SCE) anda piece of platinum were used as the reference andauxiliary electrodes, respectively. Corrosive wearlosses of the steel were determined by profiling weartracks taken from six equidistant positions along theperiphery of the ring specimen over the potentialrange from –l,800 mVSCE to 200 mVSCE at differentloads and at a sliding speed of 0.15 m/s.

Wear tracks of the specimens were observed byscanning electron microscopy (SEM). Secondary ionmass spectrometry (SIMS) was used to determine thehydrogen penetration in the wear surface and sub-surface. Small samples (5 mm by 5 mm by 5 mm[0.2 in. by 0.2 in. by 0.2 in.) were taken from the ringspecimen before the wear test using an arc cuttingprocedure and a wire electrode. Then, they were em-bedded in their original position using a conductiveglue mixed with resin. After wear, the small sampleswere struck down and stored in liquid nitrogen assoon as possible for the SIMS test.

RESULTS

Polarization Curvesof the Steel Under Different Conditions

Figure 3 shows polarization curves of the HSLAsteel under different wear conditions. Little effect ofspeed or wear load on natural potential was found,which could be explained by active dissolution ofthe steel in 0.02 mol/L H2SO4 solution. Compari-son of Curves 1 and 2 showed the effect of therotating ring specimen on the anodic polarizationcurves without wear was not significant. However,wear showed an obvious effect on the anodic polar-ization curves (i.e., under the same anodicpotential, the anodic current increased signifi-cantly [Curve 3], which was an expression of wear

CORROSION–Vol. 53, No. 3

facilitating corrosion of the steel.) Generally, abra-sion would remove any corrosion product orpassive film and expose newly deformed materialsurface to the corrosive medium. In HSLA steel,however, a protective passive film could not form inacidic solution, where the anodic reaction:

Fe → Fe2+ + 2e (1)

is favored. In comparison to a smooth, polished andannealed surface, an abraded surface should show agreater surface area, a greater dislocation density,and a greater degree of work hardening. An abradedsurface should be in a state of higher disorder andhave a lower average atomic coordination. Thus, thehigher the physical and chemical activity of theabraded surface, the higher the probability of a reac-tion. The heat of friction, agitation of the electrolyte,and increased mass transport rates also could accel-

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FIGURE 3. Polarization curves of the HSLA steel in 0.02 mol/L H2SO4

solution: (1) steady state, (2) 0.15 m/s, and (3) 0.15 m/s and 50 N.

FIGURE 4. Dependence of corrosive wear losses on load of theHSLA steel in 0.02 mol/L H2SO4 and air: (1) in air, (2) –1,500 mVSCE,(3) –200 mVSCE, and (4) at corrosion potential (–500 mVSCE).

erate the anodic reaction rate.10 The cathodic reac-tions are:

H+ + e → H (2)

H+ Me → H × Me (3)

or

H+ H → H2↑ (4)

In a steady state, hydrogen should be adsorbedon the steel surface, and it would increase thetranspotential of hydrogen evolution. Increases in thecathodic current of the rotating specimen could beregarded as the increases in the speed of hydrogenaway from specimen surface. At certain speeds, abra-sion should increase defects of the wear surfaceindependent of the cathodic reaction. A slight in-crease in cathodic current by abrasion could berelated to penetration of atomic hydrogen into thesubsurface, as induced by surface shearing stress.

Corrosive Wear Losses of the SteelFigure 4 shows the variations of corrosive wear

losses with load under different wear conditions.The figure shows a linear relationship between wearloss and load in dry air and 0.02 mol/L H2SO4 solu-tion. Damage in dry wear was much greater than in0.02 mol/L H2SO4 solution for the same loads. Thiscould be explained by the lubricating and coolingprovided by the acidic solution.9 Anodic polalization(–200 mVSCE) accelerated the anodic dissolution pro-

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cess during friction, so the damage at a potential of–200 mVSCE was more than at the natural corrosionpotential under the same load. Generally, much lessremoval of SS was found under the cathodic poten-tials (–250 mVSCE to –1,000 mVSCE) in 0.5 mol/LH2SO4 or 69% phosphoric acid (H3PO4) solution be-cause the corrosion process was retarded completely,and damage to the SS was due only to surface shear-ing stress.11-12 However, material removal of the HSLAsteel at –1,500 mVSCE was much greater than at thenatural potential and under the same wear condi-tions. This unusual phenomenon indicated hydrogenevolution increased damage to the steel during fric-tion.

Corrosive wear losses in 0.02 mol/L H2SO4 solu-tion at various polarized potentials (–1,800 mVSCE to200 mVSCE) were measured under a load of 50 N andat a speed of 0.15 m/s after a 1-h test (Figure 5,Curve 1). Within this potential range, the shapes ofthe wear loss-vs-potential curves in acidic solutionwere similar to polarization curves of the steel. Theminimum loss of the steel occurred at the naturalpotential. In the anodic polarization range, stronganodic dissolution occurred on the wear surface, somaterial removal obviously was higher than at thenatural potential. Over the cathodic polarization po-tential range, the anodic dissolution reaction wasretarded, and damage to the steel resulted mainlyfrom mechanical shearing stress of the rider. Com-pared to results at the corrosion potential, wearlosses of the steel increased with the negative shift ofpotential in the cathodic potential range (< –500 mVSCE).This unusual phenomenon may have resulted from asynergistic effect of hydrogen-induced damage and

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TABLE 1Relative Intensity

of Secondary Ionic Current Under Corrosionand Corrosive Wear Conditions (50 N, 0.15 m/s, 1 h)

Specimen Testing Condition I 1H+ / I56

Fe+

1 Original alloy 1.72 x 10–3

2 0 mV corrosive wear 1.89 x 10–3

3 –500 mV corrosion 2.40 x 10–3

4 –500 mV corrosive wear 2.52 x 10–3

5 –1,500 mV corrosion 2.91 x 10–3

6 –1,500 mV corrosive wear 3.17 x 10–3

FIGURE 5. Dependence of corrosive wear losses on potential ofthe HSLA steel at load of 50 N: (1) in 0.02 mol/L H2SO4 and (2) in0.2 mol/L Na2SO4.

FIGURE 6. Distribution of hydrogen penetration in surface undervarious wear conditions in 0.02 mol/L H2SO4 solution: (1) –1,500mVSCE, 0.15 m/s, 50 N, 1 h; (2) –1,500 mVSCE, 1 h; (3) –500 mVSCE,0.15 m/s, 50 N, 1 h; (4) –500 mVSCE, 1 h; and (5) 0 mVSCE, 0.15 m/s,50 N, 1 h.

mechanical wear. To verify this view, wear losses ofthe steel were determined under different potentialsin 0.2 mol/L sodium sulfate (Na2SO4) solution (Figure5, Curve 2). In aerated neutral solution, the cathodicreaction process was related to the reduction of oxy-gen, so that the effect of cathodic polarization on thematerial removal was less than in acidic solutionunder the same loads and potentials.

Effect of Hydrogen on Corrosive WearPrevious works have discussed hydrogen-in-

duced brittle fracture of HSLA steel, but almost noinformation has been reported on the effect of hydro-gen on its wear resistance, especially in aggressiveenvironments. In the present work, the unusuallyhigh wear losses appeared in 0.02 mol/L H2SO4 solu-tion and in the cathodic polarization range. To betterunderstand these results, hydrogen content of thetest specimens was determined at different polarizedpotentials using SIMS (Table 1). Results were ex-pressed as the relative intensity of secondary ioniccurrent (I1

H+/I56Fe+), as measured after argon ion bom-

bardment of the specimen surface for 15 s. Theextent of penetration of hydrogen ions into the sur-face was determined by attenuation of the H+ ionintensity vs erosion time in SIMS (Figure 6). It wasremarkable that the hydrogen content under corro-sive wear conditions was much greater than undercorrosion only. The concentration obviously variedwith the negative shift of potential and wear condi-tion (Table 1 and Figure 6).

Only one hydrogen peak appeared in the speci-men without applied load. This suggested hydrogen


adsorbed mainly on the steel surface, based on therapid decay of the hydrogen content-vs-erosion timecurves (Curves 2 and 4). Two peaks of hydrogen oc-curred under the corrosive wear conditions (Curves1, 3, and 5). The first peak indicated maximum hy-drogen adsorption on the wear surface and wassimilar to the peak under corrosion conditions only.The second peak was the maximum hydrogen pen-etration into the subsurface of the worn tracks.Hydrogen content in the cathodic polarization regionobviously was higher than in the natural potential oranodic polarization region. Thus, it could be consid-ered that wear accelerated hydrogen evolution at thesurface and hydrogen penetration into the subsur-face of the worn tracks. Defects such as dislocation,voids, and microcracks resulted from deformationand could be regarded as the capture traps of hydro-gen. All of these findings verified the unusualdamage to the steel was caused by the synergy of

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(a)

(b)

(c)

(a)

(b)

FIGURE 7. Worn track of the HSLA steel: (a) 0 mVSCE, 50 N, 1 h; and(b) –500 mVSCE, 50 N, 1 h; and (c) –1,500 mVSCE, 50 N, 1 h.

FIGURE 8. Morphologies of debris: (a) at –1,500 mVSCE and (b) atcorrosion potential.

hydrogen-induced damage and wear. Hydrogen con-tent at the anodically polarized potential of 0 mVSCE

was as low as in the original specimen or at thenatural potential. Therefore, the incremental wear ofthe steel may have resulted from synergism betweenstrong anodic dissolution and wear.

Morphology of Wear Tracks and DebrisThree kinds of wear processes under different

polarization potential ranges suggested that corrosivewear occurred at the natural potential, that higherlosses resulted from the synergy between strong an-odic dissolution and wear in the anodically polarizedregion, and that unusual wear related to evolution

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and penetration of hydrogen occurred in the cathodi-cally polarized region. Morphologies of worn tracksobtained in different potential ranges under a load of50 N were identified using SEM (Figures 7[a] through[c]). The strong corrosion but without the typicalbrittle fracture of the worn track appeared at an an-odic potential of 0 mVSCE (Figure 7[a]). Corrodedparallel grooves and vertical microcracks along thesliding direction were observed at the natural poten-tial (Figure 7[b]). These microcracks probably werecaused by contact fatigue, but the obvious brittlespall and vertical crack feature could be seen besidethe parallel grooves at –1,500 mVSCE (Figure 7[c]).This demonstrated the effect of hydrogen on the wearmechanism. From the morphologies of debris, it wasfound that most of the debris obtained at –1,500 mVSCE

was of a particle-like shape and presented featuresof brittle fracture (Figure 8[a]). Most debris obtainedat the natural potential (–500 mV) was indicative ofmechanical shearing and plastic deformation (Figure8[b]).

DISCUSSION

During wear, the wear track of the specimenmust have been subjected to biaxial stress (i.e., acompressive force and a shearing force by rider).

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Local plastic deformation of the wear track was un-avoidable, while cathodic polarization of the HSLAsteel in 0.02 mol/L H2SO4 solution was actually aprocess of the electrolytic charging hydrogen to thespecimen. Figure 6 (Curve 1) shows that atomic hy-drogen adsorbing on the specimen surface diffusedand concentrated into a high-strain zone by a stress-induced action. This could be regarded as interactionof the shearing stress field and the strain field ofhydrogen in the lattice, which resulted in a stressgradient or a chemical potential gradient in the plas-tic deformation layer.13 When the hydrogenconcentration increased to a critical value, theatomic hydrogen mass formed in the plastic deforma-tion layer.14 The shearing component of the internalpressure of the atomic hydrogen mass, as in an ap-plied shearing stress field, could facilitate the motionand multiplication of dislocations in the plastic zone,which decreased the apparent yielded stress of thespecimens. Hydrogen around the dislocation couldmove with the dislocation by diffusion. Dislocationwith hydrogen piled up at the crystal boundary or atstrong barriers where hydrogen concentrated. On onehand, the concentrated atomic hydrogen in the steeladsorbed chemically in the internal boundaries, suchas the crystal boundary, and decreased the surfaceenergy of the internal boundary. On the other hand,sufficient concentrated hydrogen caused hydrogen-induced microcracks in wear tracks from pile-upstress. Finally, under the surface shearing force, thebrittle spalling occurred in the wear tracks unavoid-ably. This was the reason for the unusual materialremoval of the HSLA steel under cathodic polariza-tion in 0.02 mol/L H2SO4 solution.

CONCLUSIONS

❖ Wear facilitated penetration of hydrogen into thesubsurface of cathodically polarized HSLA steel in0.02 mol/L H2SO4 solution. Hydrogen evolution pro-


moted wear damage as the result of synergy betweenhydrogen-induced brittle spalling of the wear surfaceand mechanical wear.❖ Minimum corrosive wear occurred in 0.02 mol/LH2SO4 solution at the natural potential, under a loadof 50 N, and at a sliding velocity of 0.15 m/s.❖ For the studied HSLA steel in 0.02 mol/L H2SO4

solution, the corrosive wear damage could not bereduced with cathodic polarization.❖ Wear losses of the HSLA steel in air were muchgreater than in 0.02 mol/L H2SO4 solution.

ACKNOWLEDGMENTS

This work was supported by the National NatureScience Foundation of China under contract no.59471062.

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