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Corrosion Science. 1966, Vol. 6. pp. 389 to 393. Pergamon Press Ltd. Printed in Great Britain

SHORT CO MMUNICATIONTHE ANODIC POLARIZATION CURVES OF

IRON-NICKEL-CHROMIUM ALLOYS*K. 0sozAwA-f and H.-J. ENGELL~

Max-Planck-Institut fur Eisenforschung, Dusseldorf, GermanyINTRODUCTION

SINCE the chromium in chromium-nickel or chromium stainless steels gives a remark-able corrosion resistance to these steels, a number of electrochemical studies as to itseffect has been made systematically by many investigators.l-B However, these investiga-tions are concerned mostly with the influence of chromium on iron or steel. In thispaper we report on anodic polarization curves of five iron-nickel-chromium alloys indeaerated sulphuric acid, which were obtained by a potentiostatic method.

SPECIMENS AND EXPERIMENTALA series of iron-nickel-chromium alloys (Table 1) with about 9% nickel andchromium ranging from 3.5-19.2 % were melted in a vacuum high-frequency inductionfurnace. The ingots were forged and rolled to sheets of O-5 mm thickness. Specimensof each sheet were held for 20 min at lOWC, and quenched into water if required, to

TABLE 1. CHEMICAL ANALYSES OF THE ALLOYS INVESTIGATEDAlloy No. C Si Mn Ni Cr Fe

: 0.028.0363 0.0434 0.0565 00460~01.070.05O-010.03

O-18.130.090.080.079.85.50 354.45 bal.al.9.12 11.74 bal.8-60 16.10 bal.8.32 19.20 bal.

give martensitic, martensitic and austenitic, or austenitic structures after heat treat-ment. The specimens were polished by emery paper No. 04, and submitted topolarization measurement. The anodic polarization was measured potentiostatically in2N sulphuric acid at 25C and at 90C using an apparatus similar to the one alreadydescribed. Oxygen wa s excluded by bubbling pure nitrogen gas through the testsolution. The specimen was kept at each applied potential until the current becameconstant (in certain cases this required 15 to 24 h), and the final current value was read.

*Manuscript received 8 March 1966.tPresent address: Kawasaki/Japan, Daishigawara 7007, Nippon Yakin Kogyo Co., Ltd.*Present address: Max-Planck-lnstitut fii Metallforschung, Institut fiir Metallkunde, Stuttgart,Germany.389

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390 K. OSOZAWA and H.-J. ENGELL

The specimen dissolved very rapidly at high c.d.s (> 1 mA/cm2) and in these circum-stances the time of polarization was limi ted to 5 min.Potentials were measured with reference to a Hg-Hg,SO,-1 N H,SO,-electrodeand the measured value was converted to the standard hydrogen scale.

EXPERIMENTAL RESULTS AND DISCUSSIONPotential-current density curves of the five alloys in 2 N H,SO, at 25C and 90C

are given in Figs. 1 and 2 respectively. These curves are similar to those which Olivier4

>; 103.z5 102Ee50 IO %Cr %Ni0 354 9.65A 7.45 950I All.74 9.12l 16-10 6.60o 19.20 6.32

10-l

Potentiol Eh. mVFIG. I. Potential-current density curves in 2N H,SO, at 25C.

IOt

106

E 105s9 IO'iC2 IO

0EE 1025

IO

I

10-l -Klo Do 300

% Cr % Ni0 3.54 9.85A 7.45 9.50A Il.74 9.1216-10 6.600 19.20 6.32

Potential Eh. mVFIG. 2. Potential-current density curves in 2N &SO, at 90C.

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The anodic polarization curves of iron-nickel-chromium alloys 391obtained for iron-chromium alloys in 10 H,SO, at room temperature and show thatthe addition of 9 nickel to iron-chromium alloys has little effect on their anodicpolarization. The main influence of the addition of nickel is seen n the active region8where the maximum current necessary o passivate these alloys is smaller than that ofnickel-free alloys; in other parts of the polarization curves the addition of nickel hasa smaller effect.The highest c.d. in the active state, i.e. the passivation c.d., increases as thechromium content of alloys decreasesand reaches up to 1.4 A/cm2 in the case of3.5 : 9.9 Cr-Ni steel at 90C. In the curves with higher passivation c.d.s (Figs. 1 and2), a discontinuity is observed between the active and passive states, which is due tothe steepness f the current density-potential curve relative to the internal resistanceof the potentiostat.g As understood from Figs. 1 and 2, it is not possible o determinethe exact value of the passivation potential. The potential at the highest current densitybefore passivation, which is taken as the passivation potential, becomesmore positivewith decreasingchromium content (Fig. 3). A suddenchange of the slope of the curvesin Fig. 3 can be seenat a chromium content of 12-14 at both test temperatures.A dark thick film formed on the surface of an actively corroding specimen ear thepotential of the highest current density, but when the potential was increased itdissolved in the acid, and the specimen then became passive. If the passive c.d. isnot very high, the surface of the passive metal has a metallic lustre. The lowest cd.

700 -

600 -

500 -

E 400 -

-z 300 -

EE 200 -2a

100 -

O-

-100 -

o 2NH,SO, 90-C

-200; I I I I5 IO 15 20Cr content. oAFIG. 3. Influence of chromium content of the alloys on the potential at the highestcurrent density in the active state.

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392 K. OSOZAWA and H.-J. ENGELL

FIG. 4.

IO -

106

IO -E2 104ai 103-.zcd& IO*-LEl IO -

00 -

A IOOmVA 200mV0 3oomv8 500mV0 Highes?currentdensit (active)l Lowe.5 currentdensity passive)

.

IO-0 5 IO 15 20Cr content, %

Influence of chromium content on current density in 2NId c

H,SO, at 90C.

Cr Content, ohFIG. 5. Current density at constant potential in 2N H. O, at 90C vs . chromiumcontent.

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