Corrosion of Coated Cast Iron in Aqueous Solutions

Journal of Scientific & Industrial Research Vol. 61. September 2002, pp 705-71 I

Corrosion of Coated Cast Iron in Aqueous Solutions

Rita Mehra* and Acliti Soni

Department of Pure and Applied Chemistry. Maharshi Dayanancl Saraswati University. Ajmer 305 009 Received: 14 March 2002: accepted: 03 May 2002

The cfTcctiveness of various coatings and their combination on cast iron is studied under various experimental conditions. like period of immersion. temperature. and flow velocity of corrosive medium. using weight loss and potentiostatic polarization measur..:ments. Various coatings offer protection according to their binding capacity. porosity. environmental condi- tions. and penetration of ions to the metal surface through coating. The corrosion rate enhance with increase in period of expo- sure and flow velocity. whereas at high temperatures it does not show a sharp alteration. Negligible corrosion is observed on specimens coated entirely with rubber coating (RC). paint over reel oxide coating (RO+PC). coal tar epoxy resin (CTER). coal tar zinc rich epoxy resin (CZER), and 1/3 RC + 1/3CTER + 1/3 CZER. Reel oxide (RO) is. however, anticorrosive, providing protection in composite water studies but is unable to prevent the penetration of water. Hot galvanized coating (HGC) is found to provide better protection than cold galvanized coating. Half-coated specimens are corroded more and therefore arc not suit- able for use in water supply systems.

Introduction

Cast iron is one of the most important and cheap materials used in water supply systems in spite of its tendency to corrode in aqueous environment. Studies have been made on mi lei steel and cast iron corrosion and its mitigation by using inhibitors and coatings. Or- ganic as well as inorganic coatings are in use today to protect cast iron from deterioration. Several investiga- tors have synthesized different coatings with varying properties¹•7

• The adhesivity of the coating to the sub- strate is a factor of prime importance in assessing its protection efficiency. The surface characteristics of the substrate also have a significant role to play. Besides these, the corrosion behaviour of a coating also vary with the number of ions penetrating through it and also with the amount of water being absorbed by it. We report pro- tection efficiency offered by various coatings on cast iron in water containing salts possibly present in drinking water supplies⁸

Experimental Procedure

Material-Cast iron specimens with an area of 2x2 cm² and thickness 0.50 em having chemical composition C- 3.26, S-0.10, Si-2.25, Mn-0.59, P-0.06 per cent besides

*

iron were used. A hole was drilled near the upper edge to suspend the specimen in corrosive medium.

Test Solution- Water having 600 ppm NaCI, 600 ppm KCI, 400 ppm Na

2S0

4, 400 ppm CaCOv 120 ppm CaCI,, ISO ppm MgS0₄, 400 ppm aHC0₃, 65 ppm Na 01'

S

ppm MnS0₄, 3 ppm Pb(N0₃)

2, 5 ppm KBr, and 5 ppm Kl was used as corrosive medium for all experiments.

Coating Procedure -Surfaces of iron specimens were scratched with steel wire brush and emery paper to re- move burrs, followed by washing with solution of 0.1 per cent hydrochloric acid, 50 g/L stannous chloride and 20 g/L antimony (Ill) chloride solution and then washed and rinsed with acetone, dried, marked and weighed thereafter. Coating materials of suitable con- sistency were applied in hot sunny weather by brush on the surface of specimens, as shown in Figure I. Brush strokes were varied in direction, so that coating became uniform allover and nearly free from brush marks. Sec- ond coat was applied after drying the first coat⁹.

Results and Discussion

Ejfect of' Period oflnunersion- The corrosion rate, pro- tection efficiencies and activation energy were computed using established relations. Table I shows the extent of weight loss of bare and coated specimens. The corro-

706 J SCI IND RES VOL 61 SEPTEM13ER 2002

a _h r.

d

111 a

e _{g h}

y,

II '"

II .

. ,

v~

ltl If}

k

Figure 1- Coated specimens showing pattern of coating: (a) 13are.

(b) RO, (c) CGC. (d) HGC, (c) RC. (I) CTER. (g) CZER. (h) Y2 CZER, (i) Y2 CTER. U) 1/3 RC + 1/3 CTER, (k) l/3 RC +1/3 CTER+ 1/3 CZER, (I) RO+PC. RO-Rccl oxide: CGC- cold galva- nized coating; HGC-hot galvanized coating; RC- Rubber Coating.

CTER-coal tar epoxy resin. CTZER-coal tar zinc rich epoxy resin

sion in the twelve specimens, as shown in Figure I var- ies in the order : a> i >h > j > c > d > b > I= e = f = g = k for 24 h and 72 h and a >i >h >j >c > d >b >I >e = f =g

=k for 120 h, a> i >h >j >c >d >b >I >e >k >f = g for 192 h to 360 h. Protection efficiencies showed a reverse pat- tern. Fully coated specimens offered high protection due to no contact between metal surface and corrosion me- dium. The penetration of electrolyte ions through the covering was related to water uptake which, in turn, de- pended on the type of coating and water penetration through pores. All coatings got saturated after 72h and began to detach from the substrate clue to loss of adhe- sion, leading to penetration of ions through the coating, causing corrosion of the substrate. Figure 2 shows the difference in weight of coated specimens before and af- ter immersion prior to drying(dw) vs time taken for com- plete sorption. The state of saturation and detachment was first encountered in RO and RO+PC due to lower adhesion power and then in RC, CTER, and CZER hav- ing high adhesion power. These observations are in agree- ment with those reported in literature^10-13.

Effect of Te111perature - The effect of thermal condi- tions on corrosion behavior of cast iron and coated cast iron in experimental water for 24h immersion are shown in Table 2 along with activation energy. The order of

-- - - : - L - ;_ ~ - ...J...._ h,..._ 1-.-.-...f- rr-

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0 48 96 144 192 240 288 336 Time (h)

[ ______

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!---o--·RC CTER CZER

. -7- 1/3R+113T - · ·/ · ·1/3R+1/3T+1/3Z

·-• --1-SCZER

~I -• - Y.CTER

[--W · RO+PC

--- ^---~---

Figure 2-Shows curves between difference in weight of coated specimens before and al'tcr immersion pnor to drying vs time

k at 20°C, a > i = h > j > c > d > b > e = f = g = k =I at 35°C, a> i > h > j > c = cl > b > I > f = g

=

h > i > j > c > d > b > I > e > f > g > k at 55°C and a > h

> i > j > c > d > b > I > e > k > f = g from 65 to 80°C.

Protection efficiencies (P E) followed the reverse order.

The values of protection efficiencies were almost con- stant or had negligible increase with increase in tem- perature because of increase in· dhesion of coating ¹u;_

The decrease in P E for half coated specimens was due to rapid H~evolution at coated area of specimens, lead- ing to dissolution of uncovered area ;)f specimens^16•17• RC, CTER and CZER showed bett ,r protection effi- ciency even at high temperature. Figure 3 depicts the clw vs temperature curves. The increase in weight was possibly clue to penetration of ions through micros pores of coating. Figure 4 depicts the Arrhenius plots for bare and coated cast iron. activation energy (Ea) calculated from Arrhenius plots was maximum for CTER, CZER, and minimum for half coated specimens¹⁸•

Effect ofF/ow Velocity -The results for effect of flow velocity on corrosion behavior of bare and coated cast iron specimens are discussed in terms of weight loss and protection efficiency. The order of weight loss followed by 12 specimens is: a> i > h > j > c > d > b > e = f = g = k =I for stagnant condition and 50 rpm; a> i > h > j > c

> d > b > e > I > k > f > g for I 00 lo 300 rpm flow

Table 1- Weight loss and protection efficiency* in corrosion studies of bare and coated cast iron as a function of period of immersion in composite water at 2011° C

Weight loss (mg/dm²)

I

Time Bare RO CGC HGC RC CTER CZER ¹/2CZER Y2CTER 1/3 RC+ 1/3 RC RO+PC

h (a) (b) (c) (d) (e) (f) (g) (h) (i) 1/3 1/3 CTER (I)

CTER 1/3 CZER

(j) (k)

24 67.416 5.500 20.333 19.333 Nil Nil Nil 30.000 33.56 28.125 Nil Nil

12.66 10.28 10.78 10.60

Ius

(91.84) (6978) (7124) ( 100) (100) (100) (55.50) (50.30) (58.28)

72 210.58 20.083 64.916 62.503 Nil Nil Nil 110.166 121583 102.416 Nil 2.333

3 11.43 12 40 12.22 13.70 ls.oo ^{12.25 (100) 10.24}

19.37 (90.46) (69.17) (70 28) (100) (100) (100) (47.68) (42.26) (51.36) (98.89)

120 368.33 49.916 117.583 113.000 2.830 Nil Nil 223.00 247.16 203.166 Nil 29.083

3 12.00 14.00 113.48 10.02 16.68 14.62 13.70 100) 1112

11115 (86.44) (68.07) (69.32) (99.23) (100) (100) (39.45) (32 89) (44.84) (92.10) 192 598.08 167.416 196.333 183.750 18.833 Nil Nil 412.083 432.00 345.583 16.916 104.58

3 14.10 14.75 ls.oo ^11.40 19 80 110.16 Is 76 10.78 13.92

(72.00) (67 17) (69.27) (96.85) (100) (100) (31.09) (27. 76) (42.21) (97.17) (82.5 I) 114.72

240 761.58 297.083 302.166 249.416 54.91 Nil Nil 539.750 550.16 457.750 47.416 186.416

3 13.94 13.20 16.52 12.36 110.24 112.27 17.75 12.25 16.70

117.17 (60.99) (60.32) (67.25) (92.78) ( 100) (100) (29.12) (27.76) (39.89) (93.77) (75.52) 360 1146.0 614.666 540.583 383.000 113.416 Nil Nil 811083 828.16 774.833 109.583 467.333

83 110.17 15.90 126.75 13.37 111.12 114.60 110.32 14.00 112.08

122.46 (46.41) (52.83) (66.58) (90.1 0) ( 100) ( 100) (29.22) (27.73) (32.39) (90.43) (59 22)

*Values in l)[u·enthesis are protection efficiencies in percentage

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C/l

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rable 2---weight loss and protection efficiency* in corrosion studies of bare and coated cast iron as a function of temperature in composite water for 24 h period of immersion Weight loss (mg/dm2)/SD

:mpe- Bare RO CGC HGC RC CTER CZER ¹/2CZER !12CTER 1/3 RC+ 1/3 RC+ RO+PC

tture 1/3 CTER 1/3 CTER

c

(a) (b) (c) (d) (e) (f) (g) (h) (i) Ul ^{(k) (I)}

0 67.416 5.500 20.338 16.916 Nil l\il Nil 30.00 33.500 28.125 Nil Nil

12.66 10.32 11.45 10.60

I

(91.84) (69.78) (74.90) (100) (100) (100) (55.50) (50.30) (58.28) (100) (100)

5 80.333 7.75 24.500 20.833 Nil Nil Nil 37.41 42.666 31.666 Nil Nil

12.84 lo.5o !0.82 I uo \1.22 12.02 !1.20

(90.35) (69.50) (74.06) (!00) (100) (100) (42.91) (46.89) (60.58) (100) (100)

5 89.166 9.333 27.500 30.083 Nil Nil Nil 42.66 49.500 41.666 Nil Nil

13.22 10.27 10.94 Ius 11.76 11.60 11.54

(89.53) (69.15) (66.21) (100) (100) (!00) (52.15) (44.48) (53.27) I 00) (100)

5 98.416 11.750 30.916 26.916 1.125 Nil Nil 48.91 57.500 48.916 Nil 1.000

14.50 10.77 IJ.IO ^{11.46 10.02 12.00 12.14} Ius ^lo.oo

(88.06) (68.58) (72.65) (98.85) (100) (100) (50.29) (41.57) (50.29) (100) (98.98)

5 108.250 14.125 33.833 30.083 1.250 Nil Nil 56.250 66.00 56.250 Nil 1.125

13.86 \1.00 11.32 lo.8o lo.o5 12.12 12 60 13.12 10.04

(86.95) (68.74) (72.17) (98.84) (100) (100) (48.03) (39.03) (48.03) (100) (98.96)

5 118.333 16.500 37.080 33.126 ).250 1.000 1.000 64.000 74.12 63.000 1.000 1.250

14.71 I 1.25 12.00 Ius lo.oo lo.o5 10.02 13.40 13.30 12.90 lo.oo I o.o2

(86.05) (68.62) (72.00) (98.94) (99.15) (99.15) (45.91) (37.35) (46.76) (99.15) (98.94)

0 123.416 18.250 38.500 35.500 1.250 J.J2.'i 1.125 67.00 79.416 69.166 1.166 1.33

16.23 11.44 11.46 11.92 lo.oo lv0.02 10.04 14.00 15.62 14.14 10.04 I o.o5

(85.21) (69.20) (71.23) (98.98) (99.08) (99.08) (75.71) (35.65) (43 9~) (99.0)) (98.92)

"Values in parenthesis are protection efficiencies in percentage

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-l m

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MEIIRA & SO 1: CORROSIO OF COATED CAST IRON 709

4.5

~

3.5

r2s

! 1~

0.5 0

- R O

··o··RC -cTtR

---Q-CZER

·· · · · ~-• <> • ·1/3R+1/3T+1/JZ

· · • · ·Y.CTER

F--~--~--....--- - % CZER

0 2'5 so 7'S 1 - A O + P C

Temperature ("C)

Figure 3-Shows curves between difference in weight of coated specimens before and after immersion prior to drying vs temperature

velocity of medium. The range of protection efficien- cies in percentage, offered by various type of coatings and their combination as a function of velocity are as follows : RO -91.84 to 75.85; CGC - 69.78 to 65.68;

HGC - 71.24 to 68.53; RC - I 00 to 80.12; CTER and CZER - hundred per cent at all velocities; l/2CZER - 55.50 to 43.22; 1/2 CTER-50.30 to 39.24; 1/3 RC + II 3 CTER - 58.28 to 48.74; 1/3 RC + 1/3 CTER + 1/3 CZER - I 00 to 96.20 per cent and RO+PC - I 00 to 92.54per cent. High flow velocities of the medium in- crease the penetration of corrosive ions to the substrate surface through micropores of the coating which even- tually lead to increase in weight loss¹⁴"22 Protection elli- ciencies follow the reverse order of weight loss. The difference in weight of coated specimens before and after immersion prior to drying vs velocity is shown in Figure 5.

Potentiostatic Polari-:_ation Behaviour

Fully coated cast iron specimens have high ten- dency of water uptake as compared to partially coated.

The weight of specimens coated with RO, RC, CTER, and CZER further increase, thereby indicating more ab- sorption of water. However, after being saturated, no further uptake is possible which is reflected by the con- stant weight at high value of the parameters, i.e., tem- perature, time, and velocity. More amount of water is absorbed when temperature of the medium is raised and velocity increases as compared to immersion period.

The electrochemical parametersn²⁴, such as cor- rosion current density U,,,), corrosion potential (£.,,),

T'""'Fol C"IAno r-Anc-f--l'"t" fhrr ,R, h...-.) r·Pcict·!lnf"P nf\tPnti~l (R )

2.5 . -- - -- - - - - -- - ,

0.5

2.6

-- · ·

21 ).2

1fT X 10 I<

3.4

----RO - - C G C - - -HGC

· ·6- ·· •c

- - - - -CTER --<>--CZER

- ---1\CZER

• · · liCTER

· · ·• · · 1/lR,..1/JT ---113Q+113T+

113!

-RO+PC

Figure 4 - gives Arrhenius plots for bare and coated cast iron

4-, - - - -- - - ,

3.5

~2.5

.s

~15

--e-- RO

···6··· RC I

- - -cTffi

I

-<>-CZER

I

---1/3R+1/3T

... o-.. 1/3R+1/3T+

1/3Z .. ·• · · ~CTER -~CZER

05 - R O + P C

o~~-~-~~-~~~

0 50 100 150 200 250 300 Velosity (rpm)

Figure 5- Shows curves between difference in weight of coated specimens before and after immersion prior to drying vs velocity

and protection efficiencies (P E) are given in Table 3.

Figure 6 shows polarization curves for cast iron in ex- perimental medium at 20°C for 24 h period of expo- sure. From Figure 6, it is evident that the coated speci- mens are predominantly polarized in corrosive medium than bare specimens and hence the behaviour of cast iron is anodically controlled by the coatings. Bare ones and galvanized cast iron are polarized equally in bolh direc- tions, hence the reaction is under mixed control in the case of galvanized specimens, zinc, and lead acts as sac-

··ifi,-i,d ,.nnrlP

710 J SCI IND RES VOL 61 SEPTEMBER 2002

Table 3 - Potentiostatic polarization parameters for cast iron and coated cast iron in composite water for 24h period of immersion at 20 I I ^o C

Specimen OCP Ecorr Tafel slopes

mY _mY f3a ^f3c

m V /dee-m V /dec

Bare -560 -540 57 80

RO - 50 -50 145 170

CGC -980 -980 120 132

HGC -720 -720 130 138

RC 17 40 167 202

CTER 20 30 218 240

CZER 20 30 230 256

J/3R+I/3T -249 -240 110 120

l/3R+I/3T+i/3Z 22 30 230 250

'12 CZER -280 -260 120 150

1/2CTER -260 -220 112 144

RO+PC 18 30 220 240

Figure 6- Potentiostatic polarisation curves for bare and coated specimens for 24 h of immersion in composite water at 20°C

Conclusion

From the study it can be inferred that protection against corrosion is a function of nature of surface of the substrate, binding capacity, environmental and cli- matic conditions, coatings cast iron specimens fully coated with rubber coating (RC), paint over red oxide coating (RO+PC), coal tar epoxy resin (CTER), coal tar zinc rich epoxy resin (CZER) and their combination are

Tafel Jog fcorr Rl' Corr PE

constant /(·orr }-lA Rate (per

f3 ^{·JJA mpy cent)}

14.47 1.65 44.66 0.324 22.561

23.29 0.56 3.63 6.418 1.834 91.87 27.33 1.15 14.12 1.935 7.133 68.38 29.10 1.12 13.18 2.208 6.658 70.48 39.75 0.03 1.07 37.147 0.541 97.60 49.67 0.02 1.05 47.302 0.529 97.65 52.67 0.01 1.02 51.642 0.517 97.70 47.04 1.26 18.20 2.585 9.194 59.25 52.08 0.01 1.02 51062 0.517 97.71 24.46 1.34 20.89 1.171 10.55-'l 53.22 27.39 1.15 14.12 1.939 7.133 68.38 49.905 0.03 1.07 46.640 0.541 97.60

period of exposure have a direct relation with respect to corrosion rate, whereas temperature increase does not show a significant change.

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