A study of galvanic corrosion during coulometric dissolution of galvannealed steel

Corrosion Science, Vol. 37, No. 4. pp. 587-595. 1995

Pergamon

Copyright @ 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0010-938x/95 $9.50+0.00

A STUDY OF GALVANIC CORROSION DURING COULOMETRIC DISSOLUTION OF GALVANNEALED STEEL A. BESSEYRIAS,? Kentrc

de Recherche

F. DALARD,?

J. J. RAMEAUi

and H. BAUDINS

en Electrochimie MinCrale et GCnie des Pro&d& (URA CNRS 1212), INPG/ ENSEEG, BP 75,38402 Saint Martin d’H?res, France SSOLLAC, CED, BP 109.60761 Montatairc Cedex, France

Abstract-Galvanostatic stripping is used as a coating structure analysis technique for galvannealcd steel sheet. The thickness of the coating determined in this way is underestimated by about 20% compared to the thickness measured by chemical dissolution. This study has shown that pH, electrolyte composition and current density affect coulometric measurements. Because of additional galvanic current between zinc rich zinc-iron phases and more noble phases (iron rich zinc-iron phases), hydrogen evolves during anodic stripping. This is especially observed at low current density. Also the duration of coulometric anodic dissolution cannot give the actual coating thickness. Potentiodynamic analysis tends to show that potentials observed on galvanostatic stripping curves are the result of different iron-zinc phase layers.

1. INTRODUCTION Zinc coatings are used to protect steel sacrificially against corrosive media. Coatings obtained by annealing hot dipped galvanized steel consist of several zinc-iron intermetallic phases (r, T,, 6, 5). The phase distribution is in agreement with the zinc-iron phase diagram’ (Fig. 1); their nature and thickness influence mechanical properties, especially the powdering resistance of the material. The thickness and composition of galvanized coatings are usually determined by coulometric dissolution measurements. ‘-*Each intermetallic phase can be related to a particular dissolution potential and the duration of anodic stripping is proportional to the phase thickness. Recent studies 7,8 have given different results, indicating that each potential plateau does not correspond directly to a unique phase. The purpose of this study is to determine the mechanisms which are active during galvanostatic stripping and to investigate the influence of parameters such as pH, electrolyte composition and current density. The correct control of these variables should allow optimization of this analysis method.

2. EXPERIMENTAL 2. I

METHOD

Electrode muterials

All the samples used in this study came from the same industrial galvanncaled steel sheet. They consisted of I mm thickness discs with a diameter of 27 mm. During galvanostatic stripping, the exposed electrode area was 0.385 cm2. The coating thickness was 10 pm. Numerous intermctallic compounds r, r,, d and < are liable to grow in the coating (Fig. 2). Four electrolytes were investigated in Manuscript

received

15 June 1994, in amended

form 3 October 587

1994

et ul.

A. Besseyrias

588

Weight Fe

10

20

30

40

96 Zn 50

60

IO

SO

90

Zn

1600

418

30

40 At

Fig.

1

Fig. 2.

Zinc-iron

Schematic

50

60

70

80

% Zn phase diagram.

initial coating.

this study: (I) NaCl 3.42 M, ZnSO, 0.35 M (pH = 5): (2) N+SO, I M. ZnSOJ 0.35 M (pH = 5.5): (3) NaCl 3.42 M. ZnSOl 0.35 M. H2SOA (pH = 2.5); (4) Na7SOI I M (pH = (7). All chemical products came from Prolabo and wcrc identified as ‘RP Normapur’ quality. 2.2.

Experimetml

A Solca Tacusscl The potential of SCE). The potential Potcntiodynamic

(PRT 20-2X) potcntiostat the sample was monitored versus time was rcgistcred curves were obtained with

allowed coulomctric dissolution (galvanostatic mock). with a rcfcrence clcctrodc (saturated calomel clcctrodc. with a Sefram Scrvotrace X(timc) recording device. a Princeton Applied Rcscarch Model 273 potcntiostat and

Coulometric

dissolution

of galvannealed

steel

589

a Sefram TGM 101 recording device. NaCl 3.42 M, ZnS04 0.35 M (pH = 5.0) electrolyte was used in voltamperometric experiments under potentiodynamic pseudostationary conditions’ (scan rate v = 72 mV. h-l). The coating mass determined by chemical analysis of zinc with a spectrophotomcter after dissolution in HCI 0.6 M was 76 g.rn-‘.

3. EXPERIMENTAL 3.1,

RESULTS

AND

DISCUSSION

Coulometric dissolution

3.1.1. Znfluence of current density. The shape of the coulometric dissolution curve E(time) of the coating in NaC13.42 M, ZnS04 0.35 M (pH = 5.0) depends on the imposed current density (Fig. 3). All characteristic potential values at each plateau of the curve are shown in Table 1.

2 %

i = 1.35 mA cm-l -600

0.5

1.0

Time (h)

-1000 0

I

I

1

2

Time (h) Fig. 3. Potential-time Alectrolyte : NaCl

variation during galvanostatic stripping of galvannealed steel; 3.42 M, ZnSO, 0.35 M (pH = 5.0); -current density: (a) 7.35 mA-cmm2, (b) 3.67 mA.cm-*.

Table 1. Experimental potentials of different plateaus during stripping at various current densities in NaC13.42 M, ZnS0,0.35 I mA 7.35 3.67 1.84 0.73 0.37 0.18

cmm2/Plateau

galvanostatic M, pH = 5

A

B

C

D

E

873 900 916 930 940 960

812 834 842 856 850 856

780 804 808 818 816 820

740 764 772 784 776 780

464 512 544 576 592 638

590

A. Bcsscyrias

Table 2.

Dissolution

potential of zinc-iron phases in NaCl 3.42 M, Z&SO? 0.35 M (pH = 5) current density: 5 mA.cmW2 according to Landriault and Harrison [4]

Phases Potential

Table 3.

i(mA.

mV(SCE)

Experimental

cm-“)

m(g,cmm2)

cl al

Zinc

Zeta

Delta p

Delta c

1067

950

880

x40

electrochemical

7.35

mass of zinc coating at different M. ZnSO, 3.3.5 M (pH = 5) 3.67

1.84

63.1

71.7

Gamma 800

current

densities

0.73

57.8

1

0.37 48.8

56.4

Gamma 780

with NaC13.42

0.18 3.5.3

According to the results of Landriault and Harrison’ measurements in the same medium under a 5 mA .crne2 current density (Table 2), the A and C plateaus should correspond to the 6, and I’, or IY phases, respectively. The last plateau E should correspond to iron dissolution, that is to say the attack of the substratum. With reduction of the current density, the plateau potential is increasingly negative (Fig. 3). For a particular coating, the mass of zinc (m) per unit surface obtained with current density and dissolution time, is not constant and decreases with decreasing current density (Table 3). Every electrochemical mass m is below the 76 g. mP2 value determined by chemical analysis after chemical dissolution in HCI 0.6 M (Fig. 4, curve a). Therefore the Faraday equation is not obeyed (Fig. 4, curves b and c). During galvanostatic stripping in NaCl 3.42 M, ZnS04 0.35 M (pH = 5.0) medium pitting appears on the electrode surface. This observation confirms the aggressiveness of such an electrolyte. This one is not, in such conditions, suitable for a quantitative analysis of the coulometric dissolution curves. The potential plateaus observed in these experiments are better defined when the current density is lower (Fig. 3).

30

I 0

4

Current Fig. 3. mcihods

density

I

I

8

12

(mA cm-‘)

Experimental massm ofgalvanncalcd coating by chemical (a) and clcctrochcmical (I~). (c). (a) Chemical method: (b) NaCl 3.42 M. ZnSO, 0.35 M (pH = 5.0): (c) Na,SO, 3.42 M. ZnSO, 0.35 M (pH = 5.5).

Coulometric -400

dissolution

of galvannealed

steel

591

r h

s

;, E ? ‘Z e ij b

-600

-

-800

-

-1000

i=1,83mAcm

-2

I

I 100

0

I 300

200

Time (min) Fig. 5. Potential-time variation during galvanostatic clectrolytc: Na$O, 3.42 M, 211SO~0.35 M (pH -5.5):

3.1.2.

-

stripping current

of galvannealed steel density : 1.83 mA.cm-*.

The influence of electrolyte composition and pH.

(a) Electrolyte composition-In order to avoid chloride ions, a solution of Na2S04 1 M, ZnS04 0.35 M (pH = 5.5) was used. Current density was 1.83 mA.cmP2. The curve (Fig. 5) shows a similar shape to those recorded in NaCl 3.42 M, ZnSO, 0.35 M (pH = 5.0) solution (Fig. 3). However the dissolution time is greater and consequently the potential plateau is poorly defined. The higher stripping time is perhaps due to an increase in pH (5.0 in the first case and 5.5 in the second one). The value of m (mass of zinc per unit area) determined by galvanostatic stripping is still a function of current density (Fig. 4, curve c). Without chlorides, m is higher than with chlorides, whatever the current density value. The nature of the electrolyte obviously influences the coulometric dissolution phenomena. When zinc ions are not present in the electrolyte, the dissolution time increases and thus the coating mass measurement is more accurate (Table 4). Under such conditions, the numerous plateaus are less well defined (Fig. 5). (b) pH-Table 4 shows that the mass of coating (m) increases with increasing pH value. During coulometric dissolution, gas bubbles appear at the working electrode. This gas has been identified as hydrogen. It has also been observed that the gas production is higher when the electrolyte acidity is high. Anodic dissolution and proton reduction on cathodic zones occurred simultaneously. Finally, a current due to a natural galvanic corrosion of the material must be added to the imposed current so that the total current density is increased. Such an attack modifies the result of the dissolution time. With lower current density, the larger the difference between the total current (galvanic corrosion + dissolution) and the imposed one is increased. This means that galvanostatic stripping cannot give a quantitative result at Table 4. Na,SO, m (g cm-*) PH

Electrochemical

mass of coating Na2S04 72 6

for different

Na,SO,-ZnS03 66.5 5.5

electrolytes

(i = 1.83 mA.cm-*)

NaCI-ZnS04 57.8 5

NaCl 64.1 5.5

A. Bcsscyrias (v al.

Current Fig. 6.

Relationship

bctwccn

density

corrosion

(mA cmm2)

current

during galvanostatic

(galvanic

current)

w

current

density

stripping of coating.

low current density, mainly because of galvanic coupling between zinc rich and iron rich zinc-iron phases. During coulometric dissolution, localized attack of the coating appears. This phenomenon seems to be induced by localized corrosion or (perhaps and) by coating decohesion due to hydrogen evolution giving rise to the loss of metallic particles from the galvannealed layer. Such physical behavior is not taken into account in the electrochemical measurements and falsifies the final result. The thickness of the industrial coating corresponds to a 76 gem-’ zinc mass per unit area. Thus, the corrosion current due to proton reduction can be estimated by use of this value. This method allows the relationship between imposed current density and percentage of corrosion current vs total current density (Fig. 6) to be shown. 3.1.3. The evolution of rest potential ufter partial dissolution. The rest potential was registered after partial dissolution of the first plateau (Fig. 3(a), A) in NaCI 3.42 M, ZnSOj 0.35 M (pH = 5.0) electrolyte under a 7.35 mA.cm-’ current density. The rest potential evolves continuously with time (Fig. 7). The potential plateaus appeared similar to those obtained under galvanostatic stripping. These results confirm the existence of galvanic coupling between different phases of the coating and show that galvanic corrosion is important in such an electrolyte. In Na2SOI I M,

VJ i=o,/y a

300

500

700 Time

Fig. 7.

Rat

potential

,

in NaCl 3.42 M. ZnSO,

900

(min) 0.35 M (ptl

until A (first plateau).

= 5.0) nftct- partial stripping

Coulometric

a

500

dissolution

600

of galvannealed

1700 1800

steel

593

3900 4000

Time (min) Fig. 8.

Rest potential

in Na,SO,

3.42 M, &SO, 0.35 M (pH = 5.5) after partial stripping until B (second plateau).

ZnSO, 0.35 M (pH = 5.5) the rest potential (Fig. 8) is registered after partial the condition corresponding to dissolution lasting 20 min under 7.35 mA.cm-2, point B (Fig. 3(a)). The potential of the first two plateaus are in agreement with those recorded in NaCl 3.42 M, ZnSO, 0.35 M (pH = 5.0) solution. The kinetics of dissolution or corrosion are far lower in this case. After 4000 min, the potential showed corrosion conditions corresponding to those of plateau D. 3.1.4. Surface characterization. The state of the sample surface after interruption of the galvanostatic stripping was analysed by SEM. This shows the initial homogeneous surface states. During coulometric dissolution cracks appear and become wider with increasing dissolution time (Fig. 9). In this way, because of the predetermined electrochemical behavior of these phases (as the iron content increases, the potential becomes more positive) the inner phases can emphasize the reduction of the proton. The feasibility of a cathodic reaction occurring with galvanic coupling is increased when the cracks approach the steel. Near the cathodic area, the corrosion

Fig. 9. Surface state of the coating after partial anodic stripping. --electrolyte -partial M, ZnSO, 0.35 M (pH = 5.0): -current density : 3.67 mA.cmm2: durarion: 20 min.

: NaC13.42 stripping

et al

A. Besscyrias

594

6I

I

-700

-1000

Potential Fig. 10

400

mV (SCE)

Potcntiodynamic polarization curve of galvannealed 3.42 M, ZnSO, 0.35 M (pH = 5.0): -Scan rate

steel. -elcctrolytc

: NaCl

: 72 mV.h-‘.

current is higher than at the surface and the coating dissolution is also higher. Localized corrosion can cause grain exposure and loosening of the coating. Potentiodynamic experiments In order to check each plateau potential observed during coulometric measurements, voltamperometric experiments were made under potentiodynamic pseudostationary conditions.” The current density/potential curve (Fig. 10) shows the existence of different peaks which are probably related to the different plateaus observed during coulometric dissolution. The rest potential of the sample was, before the beginning of the scan, 975 mV(SCE). During the experiments, three peaks appear at potentials which concur with the plateau potentials already determined in the case of a coulometric dissolution with the same electrolyte. Between the first peak and the rest potential, a ledge appears. The first peak is very close to this ledge. This fact could explain that it is very difficult to separate the first two phases usually present in galvannealed samples. This current density/potential curve tends to confirm the already supposed relationship between the dissolution potentials and the structure of the coating. 3.2.

4. CONCLUSIONS Galvanostatic stripping of galvannealed coating is an analysis method whose mechanism is more complex than that of the zinc only dissolution. Another reaction of proton reduction seems to be mainly the result of galvanic coupling between zinc rich and iron rich zinc-iron phases. The resulting current is added to the imposed one. This reduces the dissolution time. It is to be concluded that the coating thickness measured in this way is lower than the actual one. The lower the current density, the higher is the influence of this phenomenon. At high current density, electrochemical

Coulometric

dissolution

of galvannealed

steel

595

stripping gives acceptable quantitative results but the potential plateaus are less well defined. Cracks in the coating are favorable to galvanic coupling between zinc rich, and inner layer iron rich zinc-iron phases. Chloride ions give rise to the appearance of pits whose effect is similar. Moreover, a high concentration of protons favors the corrosion conditions. Finally, the use of galvanostatic stripping must be performed with a lot of precautions and the analysis of the stripping curve is certainly more complex than has been previously thought.

REFERENCES 1. W. G. Moffatt, in The Handbook of Binary Phase diagrams, Vol. 3, pp. &78. Gcnium Publishing Corporation (1987). 2. P. Lucas, D. Quantin and C. Brun, Galvatech 89, p. 138. ISIJ, Tokyo, Japan (1989). 3. S. Mathieu and B. Fenaille, Bull. Cercl. Etud. Mer. 14, 257-273 (1981). 4. J. P. Landriault and F. W. Harrison, CIM Bull. 71.71-78 (1987). 5. R. Hollub and Z. Jojko, Povrchove Upravy 21,35-39 (1981). 6. L. Cieslak, J. Szota and J. Boba, Ochrona Przed Korozja 23,221-225 (1980). 7. H. Baudin, A. Besseyrias and F. Dalard, Gulvatech ‘92, 2nd Int. Conf. olt Zinc and Zinc Alloy Coated Steel Sheet, Amsterdam. The Netherlands. September 8-10 (1992) - pp 455460, Centre de Rechcrche MCtallurgique, Stahl Eisen. 8. X. G. Zhang and I. C. Bravo, Corrosion 50,308-317 (1994). 9. Southampton Electrochemistry Group, in Instrumentul Method in Electrochemistry. Ellis Horwood Ltd, UK (1985).