Effect of surface roughness on early stages of pitting corrosion of Type 301 stainless steel

Effect of surface roughness on early stages of pitting corrosion of Type 301 stainless steel

CorrosionScience,Vol. 39, No. 9, pp. 1665-1672,1997 0 1997Uscvier 8cienccLtd Printed in Great Britain. All rights rcsc.& 001&938X/97S17.00+0.00 PII: ...

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CorrosionScience,Vol. 39, No. 9, pp. 1665-1672,1997 0 1997Uscvier 8cienccLtd Printed in Great Britain. All rights rcsc.& 001&938X/97S17.00+0.00

PII: soo10-938x(97poo72-3

EFFECT OF SURFACE ROUGHNESS ON EARLY STAGES OF PITTING CORROSION OF TYPE 301 STAINLESS STEEL T. HONG* and M. NAGUMOt * R and D Laboratories, Nippon Kinzoku CO- LTD, Funato, 4-10-l. Itabashi-Ku, Tokyo, 174 Japan t Department of Materials Science and Engineering, Waseda University, Ohkubo, 341, Shinjuku-ku, Tokyo, Japan Abstract-In 0.5 M NaCl solution, the early stages of pitting corrosion of Type 301 stainless steel which is wetground on silicon carbide papers ranged from 240 grit to 4OO,gOO,lOOOand 1500 grits have been studied by AC impedance method. It is found that the Warburg impedance coefficient (a), which is calculated from Nyquist impedance plots, decreases with increasing grit number of silicon carbide paper for final surface grinding. This is because that the number of metastable pits on a smooth surface is less than that on a rougher surface. The change in ,?$, (the potential at which metastable pit or pits start to grow on the steel) relates with the change in grit number of silicon carbide paper. The higher the number of the silicon carbide paper, the higher the I?,,,value. This fact suggests that metastable pit or pits starting to grow on the smoother surfaces is more difficult than that on rougher surfaces. 0 1997 Elsevier Science Ltd Keywords: A. stainless steel, B. polarization, B. EIS, C. pitting corrosion.

INTRODUCTION It is known that pitting corrosion of stainless steel is affected by HN03 treatment. Shibata and Takeyama ’ reported that the pitting potentials of Type 304 and Type 316 stainless steels increase with increasing the amount of chromium in the surface film formed by HN03 treatments. Similar works for Type 430 stainless steel was done by the authors.2 It was also found that the pitting potential (J!$,) of Type 430 stainless steel correlate with the total amount of chromium concentration (27~) in the surface film. Barbosa and co-workers3*4 found that immersion in the nitec acid solution results in the removal of sulphide inclusion, thus eliminating the most susceptible sites for attack. All of the results above indicate the effects of either the enrichment of Cr concentration in the film or removal of inclusions caused by HN03 treatment on pitting corrosion of stainless steel. Roughness of the metal surface, however, also is a major influence on pitting corrosion.s*6 Sasaki and Burstein6 pointed out that the pitting potential of stainless steel is lower for rougher surfaces than for smoother ones. Burstein and PistoriusS focused on the effect of roughness of surface on metastable pitting of stainless steel. Their results indicated that a smoother surface finish reduced the incidence of metastable pitting substantially by reducing the number of sites capable of being activated into metastable pit growth. In the present paper, the early stages of pitting corrosion of Type 301 stainless steel with different surface roughness was discussed by using a method described earlier’ (AC impedance method). The relationship between the diffusion impedance coefficient o and Manuscript received 14 February 1997; in amended form 23 May 1997 1665

T. Hong and M. Nagumo

1666 Table

I. Chemical composition

of Type 301 stainless steel (mass%)

c

si

Mn

S

Ni

Cr

P

N

0.100

0.620

0.790

0.003

6.580

17.000

0.027

0.057

surface roughness was obtained, and the potentials (E,,,) at which the metastable pit or pits start to grow in the different surfaces were estimated. EXPERIMENTAL

METHOD

The steel studied was a bright annealed Type 301 stainless steel, and the chemical composition is shown in Table 1. The specimens were cut into test pieces 50 mm long and 15 mm wide. The surfaces of specimens were mechanically wet-ground on silicon carbide paper from grit number 240 to grit numbers 400, 800, 1000, and 1500 (the smaller number representing the rougher finish, and the larger the smoother). The 0.5 M NaCl test solution was prepared from distilled water with a specific electrical conductivity less than 2 x lo-% cm-’ and reagent grade NaCl. The solution was deaerated with high purity Ar before testing and kept under an Ar atmosphere during testing. The AC impedance measurements were conducted from - 270 mV toward the positive direction at intervals of 20 mV in 0.5 M NaCl test solution for each specimen, and carried out after the specimens had been pre-passivated in the same solution for 30 min without application of potential to allow a uniform passive film formation on the surface. The impedance measurement was taken immediately after the potential was applied to a specimen. An impedance measurement system comprising a Potentiostat HA-5OlG (Hokuto Denko Ltd), PRE Analyzer S-5720C (NF ELECTRONIC INSTRUMENTS), and Computer (NEC) PC-9801 was used. A perturbation AC potential of amplitude 10 mV was applied over the frequency range from 0.5 mHz to 1 kHz. The AC impedance measurements were performed at room temperature (25 “C and all potentials recorded in this paper were referred to the saturated calomel electrode (SCE). The exposed electrode surface area was 1 cm2, and the counter electrode was an untreated Pt sheet of dimensions of 10 mm x 10 mm. EXPERIMENTAL

RESULTS

AND DISCUSSION

Polarization curves of the Type 301 stainless steel by finally wet-grinding with four grit numbers of silicon carbide papers are given in Fig. 1. It was found that the pitting potential Ep (in this paper, Ep is obtained from the anodic polarization curves on which the anodic currents increase to 10 uA/cm2) becomes more positive as the grit number of silicon carbide paper becomes larger. This fact shows that pitting potential of stainless steel is lower for rougher surfaces than for smoother ones. Similar results for Type 304 stainless steel were obtained by Burstein and co-workers.5,6 Figure 2 shows the Nyquist impedance plots of Type 301 stainless steel by wet-ground finally on silicon carbide paper of grit number 400 measured at different lower potentials in passive region in 0.5 M NaCl solution. It is found that the Nyquist impedance plots is a semicircle at -270 mV. When the potential increases above -250 mV, a diffusion tail

1667

Pitting corrosion of Type 301 SS

Grit number of silicon carbide paper -400

---800

-

1000

-

-1500

ti

0.01

-300

-200

-100

0

100

200

300

400

Potential ( mV vs SCE ) Fig. 1. Polarization curves of Type 301 stainless steel ground on silicon carbide papers to grit numbers 400,800,1000 and 1500.The arrows mark the pitting potential, Ep. Electrolyte: 0.5 M NaCl solution. Potential scanning rate: 5 mV/min.

10 Potential ( mV vs SCE ) _ d-

8

-270 ._pJ)

/’

c’

.__P.. -230

--m-

/ti’

-210

G6 2 “0 A x

4

2

0 0

2

4

6

8

0

10

a( 1060hm) Fig. 2. Nyquist impedance plots at different low potentials in 0.5 M NaCl solution for Type 301 stainless steel ground on silicon carbide papers from grit number 240 to 400. 0: diffusion tail begins to be inclined at an angle of 45” to the rr axis.

1668

T. Hong and M. Nagumo

Potential ( mV vs SCE ) d

-250

- -.-230

0

2

4

6

8

10

a( 1O”ohm) Fig. 3. Nyquist impedance plots at different low potentials in 0.5 M NaCl solution for Type 301 stainless steel ground on silicon carbide papers from grit number 240 to 800. 0: diffusion tall begins to be inclined at an angle of 45” to the a axis.

begins to appear at low frequencies (below 0.0007 Hz), and the diffusion tail becomes inclined at an angle of 45” to the a axis. This means that the diffusion reaction has taken place on the passivated electrode at the potentials above -250 mV.7 The Nyquist impedance plots of the steel by wet-ground finally on silicon carbide paper of grit numbers 800, 1000, and 1500 measured in 0.5 M NaCl solution at low potentials in the passive regions are given in Figs 3-5. For each specimen, it can been seen that there is a potential above which the diffusion tail appears on the Nyquist impedance plots, i.e. - 230 mV (grit number 800), -210 mV (grit number lOOO), and - 170 mV (grit number 1500). The Warburg impedance coefficients cs can be obtained from eqn (1)8 by using the Nyquist impedance plots of Figs 2-5 at low frequencies where the diffusion tails are inclined at the angles of 45” to the a axis (see Fig. 6). o = be_)‘/?

(1)

where b = reactive component of impedance (Ohm) at which the diffusion tail begins to be inclined at an angle of 45” to the a axis, o:2nfrad s-l), i.e. f: a frequency at which the diffusion tail begins to be inclined at an angle of 45” to the a axis. The values of o for Type 301 stainless steel by wet ground with different grit numbers of silicon carbide paper in 0.5 M NaCl are shown in Table 2. Figure 7 shows that o decreases with increasing the grit number of silicon carbide paper

Pitting corrosion of Type 301 SS

1669

10

Potential ( mV vs SCE ) 8 v -230 --D-.-210 Z6 8

---B-.

-190

---a-

-170

“0

p”

4

2

0 0

2

4

6

8

10

a( 1060hm) Fig. 4. Nyquist impedance plots at different low potentials in 0.5 M NaCl solution for Type 301 stainless steel ground on silicon carbide papers from grit number 240 to 1000.0: diffusion tall begins to be inclined at an angle of 45” to the a axis.

for final surface grinding at a given potential. According to the work of Burstein and Pistorius,’ surface roughness has a strong effect on the number of sites available for metastable pitting, the smoother surface results in the fewer number of sites capable of being activated into metastable pit growth. So the total number of the surface sites available for

Table 2.

Potential mV (WE) - 270 -250 -230 -210 -190 -170 -150 -130

The values of u (F (kOhm s”‘) its 0.5 M NaCl solution for Type 301 stainless steel ground on different grit numbers of silicon carbide papers No. 400

No. 800

No. 1000

No. 1500

0

u

(5

o

0

0

0

0

119 265 356

0 120 225 318 -

0 0 124 288 358 -

0 0 0 0 147 295 345

-

T. Hong and M. Nagumo

1670

10 Potential ( mV vs SCE ) h-190 - d-

8

._I70

- - -# - 1-150 --w-

.’

-130

/

,d .

2 % “0

6

z

4

2

t / b

0 0

2

4

6

8

10

a( 1O”ohm) Fig. 5. Nyquist impedance plots at different low potentials in 0.5 M NaCl solution for Type 301 stainless steel ground on silicon carbide papers from grit number 240 to 1500. 0: diffusion tail begins to be inclined at an angle of 45” to the a axis.

a (ohm) Fig. 6.

Calculation of a from Nyquist impedance plots.

Pitting corrosion of Type 301 SS

1671

Q250 -wJ .z B V b

200 150

silicon carbide paper

-300

-250

-200

~400

l

A 1000

0 1500

-150

800

-100

-50

Potential ( mV vs SCE ) Fig. 7. The relationship between the potential and Warburg diffusion coefficient (or) for Type 301 stainless steel ground on silicon carbide papers from grit number 240 to different grit numbers.

metastable pits on the electrode at a given potential decreases with increasing the grit number of the silicon carbide paper for final surface grinding (the larger grit number of silicon carbide paper representing the smoother finish). Since the metastable pits continue to survive depending on the maintenance of effective diffusion provided by the salt films over the mouths of these pits,“’ decreasing the number of metastable pits results in decreasing the number of the salt films forming on these pits. In another word, the number of diffusion barriers provided by the salt films decreases with increasing the grit number of silicon carbide paper for final surface grinding, therefore, leading to smaller o values. In Fig. 7, the lines through the points, which are measured in 0.5 M NaCl solution for the steel ground by silicon carbide paper with final grit numbers 400, 800, 1000 and 1500, intersect the potential axis at -268, -254, -234 and -201 mV. These potentials have been considered as the potentials (E,) at which the metastable pit or pits start to grow on the surface of steel.’ The relationship between the grit number of silicon carbide paper and E, is shown in Fig. 8. It can be observed that E,,, becomes more positive by increasing the grit number of silicon carbide paper for final surface grinding. This fact implies that metastable pit or pits starting to grow on the smoother surfaces is more difficult than that on the rougher

surfaces.

CONCLUSIONS For Type 301 stainless steel, which is ground with silicon carbide papers from grit

T. Hong and M. Nagumo

1672

,--.

-180

m

‘;a

-230 n

-270 0

500

IO00

1500

2000

Grit number of siltcon carbide paper Fig. 8.

The relationship

between the grit number of silicon carbide papers emery for final surface grinding and & of Type 301 stainless steel.

number 240 to grit numbers 400, 800, 1000 and 1500, at low potentials in passive regions in 0.5 M NaCl solution, the following conclusions are drawn. (1) When the diffusion process begins to occur at the surface of the steel, the Warburg impedance coefficient in NaCl solution at a given low potential in passive region decreases with increasing final grit number of silicon carbide paper for surface grinding. Decrease of Warburg impedance coefficient implies that the number of the surface sites available for metastable pits on the surface is decreased. (2) The potential (E,) at which the metastable pit or pits start to grow on the surface depends on surface roughness. The smoother the surface, the higher the I&, values, showing that the metastable pit or pits starting to grow on the surface becomes more difficult. REFERENCES 1. T. Shibala and T. Takeyama, in Proc. 19th Symp. on Corrosion and Protection, Japan Society of Corrosion Engineering, 1978, p. 23. 2. T. Hong, T. Ogushi and M. Nagumo, Corros. Sci. 38, 881 (1996). 3. M.A. Barbosa, Corros. Sci. 23, 1293 (1983). 4. M.A. Barbosa, A. Garrido, A. Campilho and I. Sutherland, Corros. Sci. 32, 179 (1991). 5. G.T. Burstein and P.C. Pistorius, Corrosion 51, 380 (1995). 6. K. Sasaki and G.T. Burstein, Corros. Sri. 38, 2111 (1996). 7. T. Hong, G.W. Walter and M. Nagumo, Corros. Sci. 38. 1525 (1996). 8. G.W. Walter, Corros. Sci. 26, 68 1 (1986). 9. P.C. Pistorius and G.T. Burstein, Corros. Sri. 33, 1885 (1992). IO. G.T. Burstein, P.C. Pistorius and S.P. Mattin, Corros. Sri. 35, 57 (1993). I I. P.C. Pistorius and G.T. Burstein, Corros. Sci. 36, 525 (1994).