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Corrosion cience,Vol. 39,No. 3, 453-463, 1997p.0 1997ElsevierScienceLtdPrinted in Great Britain. All rights reserved0010-938X/97 17.00+0.00

PII: SOOlCr938X(96)00122-9

THE CORROSION BEHAVIOUR OF AISI 304L AND 316LSTAINLESS STEELS PREPARED BY POWDER METALLURGYIN THE PRESENCE OF ORGANIC ACIDSE. OTERO, A. PARDO, M. V. UTRILLA, F. J. P&REZ and C. MERINO

Department of Materials Sciences, Faculty of Chemical Science, Universidad Complutense de Madrid, Av.Universitaria s/n, 28040 Madrid, Spain

Abstract-The corrosion rates of AISI 304L and 316L stainless steels prepared by powder metallurgy (P/M) havebeen studied by continuous current electrochemical methods, in organic acid solutions (acetic, formic, lactic andoxalic) at different concentrations. For comparison purposes a simultaneous study was carried out on cast AISI304L and AK1 316L steels of similar composition. For this investigation polarization resistance was the techniquechosen and values for the Tafel slopes and the StemGeary constant B were obtained. The sintered AISI 304L andAISI 3 16L steels had the highest corrosion rates. Subsequently, a kinetic study of the corrosion process has beencarried out. A crevice corrosion mechanism for the materials tested in organic acid has been suggested inaccordance with the results obtained for the P/M materials. The crevice attack is localized in the pore areas, close tothe powder particle contact zone. 0 1997 Elsevier Science Ltd. All rights reservedKeywords A. stainless steel, B. weight loss, B. polarization resistance, C. crevice corrosion.

INTRODUCTIONThe demand for P/M stainless steels has increased notably, this is fundamentally due to theirgood mechanical properties, ease of fabrication and the economic advantages they offer(especially where large numbers of complex objects are involved). The principal limitationof these materials, when compared with their cast counterparts, is their lower corrosionresistance. This is due to porosity which increases the area available for reaction.*- Thevariables of the fabrication process such as atmosphere, temperature, sintering time arebeing optimised, and the addition of alloying elements to the steels to increase corrosionresistance is being examined.-

The organic acids belong to a group of products more frequently used in the chemicaland food industries. These acids are fundamentally used as precursors for other substances.They are classified as weak acids as H + dissociation is slight in aqueous solutions. As ageneral rule the corrosivity increases when the alkyl group, attached to the acid, decreases insize. Formic acid has the largest dissociation constant and consequently is more corrosive.Numerous investigations on the corrosion behaviour of cast austenitic stainless steels existin the literature,i5 but there are very few concerning P/M stainless steels. At lowtemperatures conventional austenitic stainless steels can be used in these environments. Inthe present work electrochemical methods have been used to study the corrosion behaviourof, and evaluate the effect of porosity on, AISI 304L and 3 16L stainless steels when exposedto different concentrations of organic acids at room temperature.

Manuscript received 3 April 1996; in revised form 14 July 1996.453

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454 E. Otero rr al

EXPERIMENTAL METHODMaterials

This work was carried out on cast and P/M AISI 304L and AISI 316L austenitic stainlesssteels. Their chemical compositions are shown in Table 1.The P/M steels were obtained as follows: 15 g of prealloyed powder was uniaxiallycompacted at 700 MPa using zinc stearate as the matrix lubricant. Green densities of 6.40and 6.43 g/cm were obtained for AISI 304L and 316L, respectively. Sintering was carriedout in a vacuum furnace at 1603 K, with a vacuum pressure ~0.13 Pa, for 30 min. Theheating and cooling rate was 5 K/min. The products were 25 mm diameter, 4.9 mm thickdiscs with densities of 6.90 and 7.03 g/cm3 for AISI 304L P/M and AISI 316L P/M,respectively.The cast steels were prepared by smelting two 40 kg ingots, of the desired compositions,in an induction furnace. The products were machined into specimens of dimensions15mmx15mmx4mm.

Solutions of the organic acids were prepared using 1, 25 and 50 wt% concentrations,except for oxalic acid which used concentrations of 1, 10 and 25 wt% (the last solution waspractically saturated).Electrochemical testsA three-electrode cell was used to obtain electrochemical measurements. The workingelectrode was the chosen steel, the counter platinum and the reference a saturated calomelelectrode (SCE). An AMEL potentiostat model 551, incorporating a model 567 functiongenerator and model 500 recorder, was used to measure and register the results.A potential of &-10 mV was applied above the corrosion potential (scanning rate 2 mV/s) for different testing times. The Tafel slopes were established from the active region of thecorresponding anodic and cathodic curves. They were plotted by applying a ramp of+ 100 mV to the signal generator with a polarization rate of 4 mV/s. When R, was knownthe Stern-Geary equation was applied to obtain i,,,,. Table 2 shows the values of B for thematerials studied. The total test time was 360 h. Duplicate specimens were used in each testto show reproducibility. The corrosion rate (I/,,,,) was determined using Faradays law(ASTM G-102).16 At the end of each test, atomic absorption spectroscopy was used todetermine the dissolved elements in the electrolytic solution and calculate the equivalentweight (IV,,). When the tests were complete the kinetic laws governing the corrosion foreach experiment were calculated.

Table I. Chemical composltmn of the stainless steelsElement (wt%)

MaterialCast AISI 304LCast AISI 316LAISI 304L P/MAISI 316L P:M

< Si MIl Cr Ni MO P s0.018 0.83 0.76 18.4 11.0 0.01 0.006 0.0040.01 I 0.75 0.14 17.1 12.9 2.33 0.007 0.0060.017 0.62 SO.05 18.2 11.5 0.024 0.0070.023 0.82 0.18 16.3 13.8 2.15 0.018 0.007

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Corrosion behaviour of P/M stainless steels in organic acidsTable 2. Variation of the Stern-Geary constant (B) for the steels studied

455

MaterialConcentration

(%wt) AceticB (mV/decade)

Formic Lactic Oxalic Concentration(%wt)304L P/M 1 39 24 28 13 125 26 29 17 19 10

50 24 24 19 24 25316L P/M 1 39 27 33 15 1

25 36 32 32 13 1050 36 20 37 31 25

Cast 304L 1 102 81 92 13 125 75 94 75 19 1050 73 86 127 24 25

Cast 316L 1 89 87 98 15 125 66 66 126 13 1050 61 73 92 45 25

EXPERIMENTAL RESULTS AND DISCUSSIONFrom the electrochemical results, using polarization resistance, the current densities

were calculated. Figure 1 shows the evolution of current density with time for cast and P/MAISI 304L in acetic acid. It can be seen that the P/M specimens have current densities of twoorders of magnitude greater than those of the cast specimens. The corrosion rates werecalculated from Fig. 2 which represents the integration of the results, expressed as weightloss/area vs time, for cast and P/M AISI 304L in acetic acid.

0.0120

E omo100cAsT3w;SO% +PPM304;Z 0.0000

Z 00060! oB0400 0.0020

0.00000 loo 200

Time9Fig. 1. Variation of the current density as a function of time in acetic acid for P/M and cast AISI304L.

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456

PI&. 2 Variation of the corrosion rate with tune in acetlc acid far P/M and cast AN 304L.

Similarly, the corrosion densities for cast and P/M AIS1 3 16L were calculated. Figure 3shows the corrosion rate expressed as a function ofweight loss per unit area vs time, in aceticacid. Comparing Figs 2 and 3 it can be seen that P/M AISI 316L has a greater corrosionresistance in this environment than P/M AISI 304L; however, both are inferior to the caststeels. The weight losses are similar for both the cast steels and of order 20 times smallerwhen compared with the P/M AISI 304L.

The corrosion densities were measured for all the tests in all the solutions so that thecorrosion rates could subsequently be calculated.

0.50

0.40

0. 30

0.20

0.10

0.00

+ cAsT318; 1% * PM316;1%8 CAST 318; 25% * PM 316; 25%G CAST316;50% + PM316;50%

0 100 150 200 250 300 350 40Time (h)

Fig. 3 Variation of the corrosion rate with time in acetic acid for P/M and cast AISI 316L.

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Corrosion behaviour of P/M stainless steels in organic acids

25.00 8 CAST304;25% * PMm=%1 + CAST304;50% + PM304i5OK0.00

zii 10.005.00

I5.000.00

451

Fig. 4. Variation of the corrosion rate with time in formic acid for P/M and cast AISI 304L.

2500

2000 f3 CAST316;25% * PM316;25%e CAST316;50% + PM 31s;50%1.500

1.000

0.500

0.000

nrfwFig. 5. Variation of the corrosion rate with time in formic acid for P/M and cast AISI 316L.

Figures 4 and 5 show the corrosion rate, expressed as a function of weight loss per unitarea with time, in formic acid. As this is the most aggressive of the acids used, the increase inconcentration causes increases in the difference between the corrosion rates of the P/M andcast materials. Again P/M AISI 316L has better corrosion resistance than P/M AISI 304L.The weight loss per unit area was similar for both the cast steels tested.Figures 6 and 7 show the corrosion rate, as weight loss vs time, in lactic acid. Thesmallest differences between the rates for all the steels were observed in this environment.

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458 E. Otero el al.

0.60 0 CAST304;26% t PMm25%+ CAST304;60% + PM304;50%0.60

Fig. 6. Variation of the corrosion rate with time m lactic acid for P/M and cast AlSl 304L

0.60

0.60 CAST 316; 26%++ CAST318;60% + PM316;50%

0.40

200Timem

Fig. 7. Variation of the corrosion rate with time in lactic acid for P/M and cast AISI 316L

A similar behaviour is shown in oxalic acid, Figs 8 and 9, where similar rates are foundfor the P/M steels. The cast materials had lower corrosion rates than the P/M materials. Therelatively high corrosion rates observed in oxalic acid are due to the fact that the 10 and 25%tests were agitated to maintain a homogeneous dissolution.

Tables 3 and 4 show the calculated kinetic laws for all the tests; as may be seen, in allcases the kinetics follow linear behaviour. The kinetic constants calculated for the cast steelsare small and similar, as would be expected in the solutions tested at room temperature.

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Corrosion behaviour of P/M stainless steels in organic acids 4598.0007.000 0 cAsl 304;10% * PM30410~8.000 + cAsT304;25% + PM3G9;25%5.0004.0003.00020001.0000.000

0 50 100 150 ZOO 250 200 350 400

Fig. 8. Variation of the corrosion rate with time in oxalic acid for P/M and cast AISI 304L.

8.000 8 cAs1316; 10% *PM 316;10%= 50001'

+ CAST31625% + PM316;25%g 4. 000d 3. 000! i I=F 2000

1. 0000. 000

Fig. 9. Variation of the corrosion rate with time in oxalic acid for P/M and cast AISI 316L.

Comparing these results with those obtained for the P/M steels, differences of up to twoorders of magnitude in formic and oxalic acids, and one order of magnitude in acetic andlactic acids, may be observed. In general the slopes increase with increasing acidconcentration, the P/M materials showing the most significant increases. A sharperincrease in the kinetic constant values is observed for the cases which present the greatercorrosion rate (formic and oxalic), and fundamentally for P/M AISI 304L. The kineticconstants for P/M AISI 3 16L are always lower than those calculated for P/M AISI 304L.A corrosion mechanism for the P/M stainless steels, exposed to the organic acids, is

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460 E. Otero er al.

Table 3. Corrosion kinetic equatjons for the P/M steelsKinetic law

[Acid] AISI 304L P/M AISI 316L P/MAcetic acid I o/o I= -8.9x IO a 6.4x 10 / I -2.2X 10-2+9.4x lo-4r

r = 0.9954 I = 0.9947

Formic acid

2 5 % i 2.5x lob-: t I 8 x IO 7r ~~-1.6xl0~+lO-tr-0.9910 I = 0.9983

SO;0 ,-m- 0.291 6x 10. I ,=-1.8x10~+1.4x10 ?tr = 0.9865 r = 0.9926

I % y== -0.36+ 8.8~ IO-/ y= -6.9x 10m+2.1 x IO- Ir = 0.9898 I = 0.9950

2 5 O/o J : -2.68-t 8.5 x 10 r 4 -0.13-5.3x lo- ?!I = 9938 i- = 0.9966

500/o J = -2.43-i 4.2 x 101 ? -0.47+7.9x 10vJtr = 0.9848 r=0.9898

Lactic acid I % ~~--4.7xlo~+5.7xlo~4f.I< 140 h, r=0.9950;y= -0.24+ 2.Tr.r>l40h,r=0.9993

~.=-11.6x10~~+1.2x10~~tr = 0.9990

2570 11 - I .6 x IO +7.5x IO-r J= -3.8 x 10 2+1.5x lo-?/I -=0.998 I r = 0.9848

SOD/o ,= - I.2 X IO - 7.7 X IW ?( v=8.3x 10 If 5.8 X 10-41r = 0.9787

Oxalic acid I o:, I= -0.18+ lo-?1 1= 1.9X lo-+9.1 X lo-j/I z 0.9990 I = 0.9996

I O90 ,) 5x IO r7x IO :/ 1=0.28-t1.2x lo-:/r=O.Y93I r=0.9910

2 5 30 _i: --Cl.33 4 2.3 * IO :I 1.1 -0.39-1 1.7x lo-{r = 0.994 I r-0.9902

proposed based on the test results and microstructural observations. Figure 10 is aschematic diagram of the proposed mechanism.The acid penetrates into the pores [Fig. 10(a)] where it produces an elevated number of

protons, this giving rise to dissolution of the passive layer. An appreciable decrease in thepartial pressure of O2 in the crevices is produced, which prevents the formation orregeneration of the passive layers in these regions. The attack continues from the crevicealong the edges of the original powder particles, eventually interconnecting all the pores andcausing partial structural disintegration of the material. This eventually leads to generalized

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Corrosion behaviour of P/M stainless steels in organic acidsTable 4. Corrosion kinetic equations for the cast steels

461

Kinetic law[Acid] Cast AISI 304L Cast AISI 3 16L

Acetic acid 1% y= -3.8x 10 +2.2x lo-?r=0.9945 y= -4.7 x 1O-3+2.8x 10-4cr = 0.99665% y=3.ox 10 3+1.4x 10-41 y= -2.2 x 10-3+2.5x 10-42

r = 0.9984 r= 0.999450% y= -4.9x 10 + 2.6 x 10-4t

r = 0.9943y= -2.4x 1O-3+2.4x 1O-4f

r = 0.9975Formic acid 1%

25%

50%

y= -2.2 x lo-3+ 1.5 x 10-42r = 0.9999

y=-1.5x10 -=+2.9x 10-4tr=9781

y= -2.5 x 10-3+ 1.8 x 10-4tr = 0.9998

y=-7.7x10 -+1.9x lo-4tr = 0.9994

y=2.5 x 1O-3+7.3x l0-4rr=0.9980

y= -5.1 x 10 4+ 1.5 x lo-4tr = 0.9983

Lactic acid 1% y= -2.6x 10-4+ 1.3 x 10-4t y= -6.8 x 10 4+ 1.3 x 10-42r = 0.9984 r = 0.9996

25%

50%

y= -2.6x 10-3+2.1 x 1O-4rr = 0.9894

y= -6.2x 10-3+2.1 x 1O-4fr=0.9985

y= -2.6x 10 3+2.1x 10-41r = 0.9873

y= -5.7x 10 3+3.1 x lo-4rr = 0.9996

Oxalic acid 1% y=-1.2x10 -3+ 1.6 x 10-4tr = 0.9996

y= -3.7 x 10 3+3.2x 10-4tr = 0.9990

10% y= -5.9x 10 3+2.5x 10-4tr=0.9913y= -8.5x 10 3+ 3.7 x lo-4t

r = 0.993125% y= -6.6x 10 3+2.9x 1O-4rr = 0.9692

y= -2.9x 10 3+4.7x lo-4tr = 0.9892

corrosion, caused by a mechanism due to crevice corrosion [Fig. 10(b) and (c)l. This justifiesthe high corrosion rates. The effect is much more acute when the initial concentration of acidis increased.

CONCLUSIONS(1) The porosity of the P/M steels causes the corrosion rates, in the acids tested, to behigher than for conventional steels. This is due to a generalised corrosion process induced bya crevice corrosion mechanism. In the pores, high concentrations of protons are found,

producing the dissolution of the passive layer in the crevices.

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462

Fig. IO. Proposed corrosion mechanism for the P/M stainless steels: (A) acid attack; (B)interconection of pores; and (C) detail of B at higher magnification.

(2) In all test conditions P/M AISI 304L showed corrosion rates slightly higher thanthose of P/M AISI 316L.

(3) The lowest corrosion rates for the P/M steels were found in acetic and lactic acids.(4) The absence of pores in AISI 304L and 3 16L was the reason why the corrosion rates

for these materials were of two orders of magnitude lower than those of the P/M steels.Ackno~,lrdgmtmf.r--The authors wish to express their gratitude to CICYT for financial support of this work(Project MAT-0181-1993-959).

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Scotland, Vol. 3 (1994).16. ASTM Standard G102-89, Practice for calcul ation of corrosion rate and relate information f rom

electrochemical measurements. American Society for Testing and Materials, Philadelphia, PA (1989).