The 3.5wt% NaCl Aqueous Corrosion of Heat-exchange Steel ...The electrochemical corrosion behavior...

13 24 89 6

Journal of Chinese Corrosion Engineering, Vol.14 No.2, PP.13~24(2000)

The 3.5wt% NaCl Aqueous Corrosion of Heat-exchange Steel After

Hot Corrosion with Pre-coating NaCl/Na2SO

4Mixture

Hsi-Chang Ma ,Chaur-Jeng Wang , Bin-Jen Syau , Jyang-Tau Tang

SB450 STP A24 STP A26

2mg/cm2 NaCl/Na2SO

4 750 850

3.5%NaCl

icorr

(noble) STP A26 icorr

(passivation)

NaCl

icorr

2~3

icorr

NaCl

icorr

NaCl

Na2SO

4

Na2SO

4 i

corr

Fe1-X

O Fe1-X

O

icorr

Fe1-X

O

-13-

*

Material Science and Technology Center, National Taiwan University of Science and Technology

**

Department of Mechanical Engineering, National Taiwan University of Science and Technology

***

Chung-Shan Institute of Science and Technology

-14-

Abstract

The electrochemical corrosion behavior of three Fe-Cr alloys (containing SB450, STP A24, and

STP A26) after hot corrosion with 2mg/cm2 NaCl/Na2SO

4deposits at 750 and 850 were

studied in 3.5% NaCl aqueous electrolyte. The corrosion mechanism was evaluated in terms of

scale morphology by OM and SEM, XRD, and dynamic polarization curve measurements. The

results showed that the higher Cr content in alloy the lower icorr and the higher Ecorr

poetntial

which tends to be more noble. The lowest icorr

and a passivation characteristics, STP A26

displayed the best corrosion resistance of the three alloys.

Because the oxy-chlorination took place when hot corrosion with the NaCl deposits, scales

become more porous and easy to corrode, leading to an increase of icorr

by a factor of 2 as

compared to the raw materials. Thus, the reproduction of cyclic polarization curve gets worse,

and the Ecorr

decreased. On the other hand, the scales are more dense in simple oxidation, so that

crevice corrosion is more difficult to occur. As a result, icorr will be close to that of the raw

material, and the reproduction of cyclic polarization curve is second to raw materials, however the

corrosion resistance is highly improved.

As increasing temperature and exposure duration, the dominating hot-corrosion environment

change to simple oxidation in air or Na2SO

4-deposit. The overall reaction is favorable to form

dense oxide scales, which reduces the icorr

but increases the Ecorr

. However, the high-defective

Fe1-X

O scale retained on the alloy cause a significant increase in the icorr

as compare to the

specimen without Fe1-X

O.

Keywords : dynamic polarization ; NaCl aqueous corrosion.

80% [1]

[2,3] PH

89 6

-15-

[4]

[5,6]

A26 A24

SB450

[7]

[7] CNS 8696 SB450( SB450)

CNS 5806 STP A24 ( A24)

STP A26( A26) 750 850

2mg/cm2 NaCl/Na2SO

4

3.5wt%NaCl

( icorr

) ( Ecorr

)

1

2mg/cm2

100%NaCl 50%NaCl/50%Na2SO

4 100%

Na2SO

4 750 850

1 24

[7]

(Tafel)

800

[7]

100 100%

5

ASTM G5

1 9mm

60 3

3N KCl /

3N KCl 1

3.5 wt.% NaCl

300Cc.c.( /

472c.c./cm2 )

2mm

0.5mV/sec

250mV 250mV

-16-

89 6

25 1

0.5L/min

-1.35V(SCE) 5

Model 352/252

Corrosion Analysis Software V.2.01

( Ecorr

) (

icorr

) (a )

/

100mV

10-2 A/cm2

A24 850 24 800

3.5%NaCl

[7]

2 Fe2O

3Fe

3O

4

Fe1-X

O Fe1-x

S Na2SO4

SEM 2~4

Na2SO

4

NaCl

Na2SO

4

Fe1-X

O

NaCl Fe1-X

O

[7]

3.5%NaCl

(Ecorr

) (icorr

) (a )

3 NaCl/Na2SO4

3.5%NaCl Ecorr

icorr a

4 7

icorr

Ecorr

( )

Fe1-X

O

(1) Fe1-X

O icorr a

Fe1-X

O

( ) Ecorr

( ) Ecorr

(2) Fe1-X

O icorr

NaCl

1 NaCl icorr

24

Ecorr

50%NaCl/50%Na2SO

4

100%NaCl Ecorr

( )

a80

NaCl/Na2SO

4 850

1 3.5%NaCl

5 7 A26

A24 SB450

8 5 0 1

100%NaCl 50%NaCl/50% Na2SO

4

-17-

A24 850 24

[7]

Fe2O

3Fe

3O

4Fe

1-XO A24

-

539mV -653mV 8

3.5%NaCl

-533mV -

689mV

(1) (2) (3) (4)

(5)

(6)

Cr2O

3

3

3.5%NaCl icorr

A26

( )

5 7 A24

SB450

9 A26

2

Fe1-X

O

Na2SO

4

[7]

Fe1-X

O

4 7

Fe1-X

O 3.5 NaCl icorr

Fe1-X

O

icorr

Fe1-X

O

Fe1-X

O

Fe1-X

O

Fe1-X

O

[8]

Fe1-X

O

F e1 - X

O

Fe(OH)3

Fe(OH)3

Fe1-X

O 4 5

Fe1-X

O

G

( )

3.5%NaCl

8 A24 850 24

Fe2O

3Fe

3O

4Fe

1-XO

A24 -539mV

-18-

89 6

-653mV

( )

4

7 Fe1-X

O

Fe1-X

O

4 5 750

Fe3O

4

3 A26

SB450 ( )

A26

SB450

[7]

NaCl 1

3.5%NaCl

4 7 NaCl

1 icorr

2~4

A/cm2 ( 2 ) 5 7

NaCl 850 1

SB450 A26 A24

icorr

( )

24

NaCl 24

1 4 7

icorr

( )

Na2SO

4

[7]

Fe1-X

O

Fe1-X

O

4 7 icorr

Fe1-X

O

Na2SO

4

Fe1-X

O ( )

[7] 1

NaCl

100%NaCl

4 7 icorr

5 7

icorr

24

NaCl Na2SO

4

A26 Fe1-X

O

icorr

Fe1-X

O A26

icorr

1

1

4 7

100%NaCl (

) 24

Na2SO

4100%Na

2SO

4

-19-

1. 3.5%NaCl icorr

2. A26 icorr

A24 SB450

3.

( )

4. icorr

icorr

( )

5. Fe1-X

O icorr

Fe1-X

O

icorr

Fe1-X

O

6. icorr

7. NaCl

icorr

2~3

( )

8. NaCl

icorr

9. NaCl Na2SO

4

Na2SO

4

icorr

( )

10.

A26

NSC 89-2623-D-011-004

1.

(1999)

2. F.L. LaQue; in "Marine Corrosion Causes and

Prevention", John Wiley, New York (1975).

3. B.S. Phull, S.J. Pikul, and R.M. Kain; "Seawater

Corrosivity Around the World: Result from Five

Years of Testing", Corrosion Testing in Natural

Water: Second Volume, ASTM STP 1300 (1997).

4. 13 (1999) 33

5.

304

(1999) 187.

6.

ibid 45.

7.

( )

8. (1999) 17.

5 750 24 Ecorr

icorr a

Table 5 The Ecorr

, icorr

and a

of alloy after hot

corrosion at 750 for 24hr.

Corrosion Material E

corr(mV) i

corr aenviroment

Simple A26 -560.0(1.0) 1.7(0.2) 114.1(2.1)

oxidation A24 -556.1(21.9) 49.1(8.4) 317.0(36.1)

in air SB450 -547.6(19.0) 17.3(4.9) 527.1(69.2)

100/0 A26 -591.1(7.2) 2.0(0.3) 101.2(26.0)

A24 -554.9(16.0) 3.4(0.7) 69.2(8.6)

Hot SB450 -584.7(20.2) 32.0(4.7) 380.8(69.0)

corrosion 50/50 A26 -609.0(3.6) 3.0(0.5) 108.3(49.1)

(NaCl/ A24 -668.0(41.3) 32.3(3.2) 489.7(21.7)

Na2SO

4 ) SB450 -597.5(33.8) 20.4(5.1) 207.9(50.7)

0/100 A26 -599.4(4.2) 8.5(1.2) 506.8(135.0)

A24 -637.3(10.4) 46.4(3.6) 353.7(51.5)

SB450 -600.1(13.3) 33.2(7.1) 347.3(49.8)

( ) F1-X

O

Remark The value in the bracketed is the standard

deviation, the bold characters indicates the

existence of Fe1-X

O.

-20-

89 6

1 (wt%)

Table 1 The chemical composition of alloys (wt%)

Material C Mn Cr Ni Mo V Fe

A24 0.06 0.47 2.34 - 1.00 0.015 bal.

A26 0.09 0.40 8.25 0.04 0.93 0.225 bal.

SB450 0.20 0.91 - 0.10 - - bal.

2 [7]

Table 2 The corrosion products of alloys after hot

corrosion. [7]

Corrosion Material 750 850

environment 1hr 24hr 1hr 24hr

Simple A26

oxidation A24

in air SB450

A26

100/0 A24

Hot SB450

corrosion A26

(NaCl 50/50 A24

/Na2SO

4) SB450

A26

0/100 A24

SB450

Fe2O

3, Fe

3O

4Fe

1-XO

FeCr2O

4Cr

2O

3Fe

1-XS

Remark Fe2O

3, Fe

3O

4Fe

1-XO

FeCr2O

4Cr

2O

3Fe

1-XS

3 Ecorr

icorr a

Table 3 The Ecorr

, icorr

and aof raw metal.

Ecorr

(mV) icorr

((A/cm2)a

A26 -494.3(8.4) 2.5(0.3) 448.0(31.4)

A24 -694.7(6.8) 3.9(0.7) 63.7(5.0)

SB450 -710.3(5.9) 5.5(0.5) 78.1(5.2)

( )


deviation.

4 750 1 Ecorr

icorr a

Table 4 The Ecorr

, icorr

and a

of alloys after hot



corr (mV) i

corr aenviroment

Simple A26 -614.0(6.3) 2.3(0.5) 76.0(6.2)

oxidation A24 -591.1(4.6) 3.8(0.2) 61.0(6.3)

in air SB450 -587.6(5.3) 5.9(0.4) 81.3(21.0)

A26 -608.7(3.8) 3.6(1.7) 74.1(12.8)

100/0 A24 -623.0(16.3) 8.5(3.1) 63.1(3.5)

Hot SB450 -595.5(1.4) 9.8(2.2) 70.3(3.3)

corrosion A26 -660.7(7.2) 14.0(1.9) 134.3(39.5)

(NaCl/ 50/50 A24 -645.6(14.6) 9.8(1.3) 71.0(9.9)

Na2SO

4 ) SB450 -709.2(29.8) 6.2(1.5) 88.6(4.7)

A26 -600.6(14.1) 10.8(1.2) 529.5(21.5)

0/100 A24 -620.2(5.0) 28.9(2.5) 378.0(11.0)

SB450 -611.7(1.2) 11.1(3.8) 1047.2(89.4)

( ) F1-X

O


deviation, characters the bold characters

indicates the existence of Fe1-X

O.

6 850 1 Ecorr

icorr a

Table 6 The Ecorr

, icorr

and a

of alloy after hot



corr (mV) i

corr aenviroment

Simple A26 -577.6(9.8) 1.8(0.3) 84.8(10.6)

oxidation A24 -518.5(9.5) 3.1(0.3) 72.7(2.6)

in air SB450 -516.1(2.4) 22.9(1.5) 317.4(118.0)

100/0 A26 -645.4(24.1) 5.7(1.2) 107.1(24.9)

A24 -584.9(22.5) 8.4(3.1) 91.1(32.7)

Hot SB450 -594.7(4.5) 7.3(1.8) 101.3(1.7)

corrosion 50/50 A26 -724.6(26.0) 8.1(0.8) 83.8(4.0)

(NaCl/ A24 -713.7(18.7) 8.8(1.1) 127.3(6.0)

Na2SO

4 ) SB450 -578.0(58.4) 7.5(0.5) 663.7(38.1)

0/100 A26 -592.9(5.7) 4.1(1.4) 92.8(1.0)

A24 -623.8(9.5) 27.8(3.3) 212.9(16.6)

SB450 -656.3(20.7) 21.1(7.4) 297.5(38.6)

( ) F1-X

O



existence of Fe1-X

O.

-21-

1

Fig 1 The cutaway view of polarization cell.

7 850 24 Ecorr

icorr a

Table 7 The Ecorr

, icorr

and aof alloy after hot corrosion

at 850 for 24hr.

Corrosion Material Ecorr

(mV) icorr a

enviroment

Simple A26 -493.1(12.4) 2.8(0.3) 300.8(13.7)

oxidation A24 -555.3(0.6) 76.9(2.3) 436.4(49.1)

in air SB450 -596.8(3.3) 12.3(4.9) 119.7(23.5)

100/0 A26 -550.8(7.8) 2.4(0.4) 52.1(2.7)

A24-5 61.8(3.1) 97.6(6.3) 307.3(33.2)

Hot SB450 -580.4(0.3) 75.5(24.0) 97.0(1.6)

corrosion 50/50 A26 -633.3(4.1) 4.4(0.1) 63.9(1.9)

(NaCl/ A24 -609.9(3.0) 114.4(12.1) 486.4(21.6)

Na2SO

4 ) SB450 -587.5(10.5) 18.7(1.2) 209.3(17.3)

0/100 A26 -586.0(34.0) 7.2(1.0) 324.3(20.7)

A24 -604.5(15.8) 95.0(12.2) 433.8(31.7)

SB450 -586.0(19.2) 24.4(3.3) 200.3(1.3)

( ) F1-X

O



existence of Fe1-X

O.

-22-

89 6

3 A24 1

SEI (a) 750

(b) 850 (c) 750

100%NaCl (d) 850 100%NaCl (e)

750 50%NaCl/50% Na2SO4 (f) 850

50%NaCl/50%Na2SO

4

Fig 3 SEM micrographs of the surface morphology of

A24 after hot corrosion for 1hr. (a) simple

oxidation at 750 , (b) simple oxidation at 850 ,

(c) hot corroded at 750 with 100%NaCl pre-

coatings, (d) hot corroded at 850 with

100%NaCl pre-coatings (e) hot corroded at 750

with 50%NaCl/50%Na2SO

4pre-coatings, (f) hot

corroded at 850 with 50%NaCl/50% Na2SO

4

pre-coatings.

2 A26 1

SEI (a) 750

(b) 850 (c) 750

100%NaCl (d) 850 100%NaCl (e) 750

50%NaCl/50% Na2SO

4(f) 850

50%NaCl/50%Na2SO

4


A26 after hot corrosion for 1hr. (a) simple








4

pre-coatings.

-23-

4 SB450 1

SEI (a) 750

(b) 850 (c) 750

100%NaCl (d) 850 100%NaCl (e) 750

50%NaCl/50% Na2SO

4(f) 850

50%NaCl/50%Na2SO

4


SB450 after hot corrosion for 1hr. (a) simple








4

pre-coatings.

5 A26 850 1

Fig 5 The cyclic polarization curve of A26 hot

corroded at 850 for 1hr.

6 A24 850 1

Fig 6 The cyclic polarization curve of A24 hot


-24-

89 6

7 SB450 850 1

Fig 7 The cyclic polarization curve of SB450 hot


8 A24 850 24

1 0 0 %

3.5%NaCl

Fig 8 The corrosion potential (Ecorr

) from the outermost

oxide scale to alloy matrix after appropriate

abrasively for A24 after simple oxidation at 850

for 24hr.

9 A26 3.5 wt % NaCl

Fig 9 The pitting morphology of A26 after cyclic

polarization curve test in 3.5wt% NaCl aqueous

electrolyte.

The 3.5wt% NaCl Aqueous Corrosion of Heat-exchange Steel ...The electrochemical corrosion behavior...

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