A Case Study of Stainless Steel Water Supply Pipe Corrosion Caused by Weld Heat Tint

7/27/2019 A Case Study of Stainless Steel Water Supply Pipe Corrosion Caused by Weld Heat Tint

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A CASE STUDY OF STAINLESS STEEL WATER SUPPLY PIPECORROSION CAUSED BY

WELD HEAT TINT


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Amongst

several techniques, pickling using a mixture of HF and HNO

3

acid has been found to be the most

effective method[7, 9 11]. However, pickling is not always possible due to safety issues,

cost andaccess. Such problemsaremore prominent for site-welded stainless steel systems as it is

relatively

easier to control welding parameters and pickling processes in the workshop. Thus, it is

important

that end users and fabricators have a better understanding of the effect of heat tint on

localised

corrosion for stainless steel systems and its working environment. An agreement on

acceptableheat tint level should be finalised between fabricators and end users prior to

manufacturing and it should be welldocumented.

An acceptable heat tint level can be determined by heat tint colour reference charts, such asAmerican Welding Society Standards AWS D18.1/D18.1M (2009) [12] and AWS D18.2

(1999) [13]

or Force charts Nos. 1 -5 (The Force Institute, Denmark) [14]. For austenitic stainless steel

(e.g.

AISI Types 304 and 316) welds used in food, dairy and pharmaceutical industries, both

colour

charts suggest that the acceptable heat tint level should not go beyond grey or straw yellow

(e.g.

brown, blue or black). Above this level, the weld may be susceptible to localised corrosion

attack.

When unacceptable heat tint is unavoidable and pickling is not an option, fabricators and end

users should consider carrying out corrosion tests on welded samples to determine if the weld

quality willmeet the design specification.


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(

b)


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Figure 3: Internal views of pipes with weeping issue. (a) Shorter pipe with butt weld;large

corrosionnodules were observed at the weld root.(b) Longer pipe with weeping along the pipe length.

The

location of the suspected seam weld is indicated.

The corrosion product nodules were found to be quite porous and fragile (Figure 4a) and

some

large pits were observed underneath the corrosion products (Figure 4b).

Figure 4: Detailed views of corrosion product nodules and underlying metal appearance. (a)

Porous corrosion product nodule formed on the ID surface of a longer pipe sample and (b) pit

(~ 0.75 mm in diameter, arrowed) observed underneath the corrosion product from the ID

surface

of a shorter pipe sample.

Localised areas associated with the pitting at the pipe inner surface were cleaned using a

cotton

swab doused in isopropanol. In both cases of pitting associated with a butt weld and along a

suspected seam weld, a dark brown coloured heat tint was identified (see Figure5). The heat

tint

also confirmed the presence of a seam weld in the longer pipe section samples.

Several metallurgical samples were removed across the pitting damaged area including

associated

weld from both short and long pipe section samples. In each case, significant pitting was

observed

adjacent to the weld (i.e. in the parent metal or heat affected zone) just beneath the pipe inner

surface,with the largest pit sampled having progressed almost entirely across the pipe

wall. No

pitting of the weld metal was identified. The corrosion products formed above and within thepit

were porous and possibly multiphase. A typical example of the pitting damage is shown in

Figure 6.

(a)

(b)

Suspected seam weld

(a)

(b)


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Figure 5: Appearance of (a) pipe with butt weld and (b) pipe with suspected seam weld afterlocalised cleaning about the pitting regions. In both instances a heat tint (arrowed; dark

brown

discoloured region) was identified.

Figure 6: Macrograph showing a typical cross-section view of pitting damage near the weld.

Sample sectioned from a longer pipe sample with seam weld. Marker division in mm.

2.2

Root cause of the failure

Examination of the supplied pipe samples revealed that weeping was caused by pitting

damage

initiated from the pipe ID at weld heat tint regions. Crevice corrosion attack initiated at the

weld

heat tint regions and progressed further as pitting corrosion. The species responsible for

crevice

and pitting corrosion is usually chloride ions; these can penetrate the passive film that

stainless

steels rely on for their excellent corrosion properties. The action of the ions cause local

destruction

of the passive film and prevents repassivation, allowing rapid dissolution of the metal to

proceed.

The resistance to pitting and crevice corrosion is related, and for Type 304 and 304L,

adequate

pitting resistance is expected when fully immersed in domestic water containing < 200

ppm Cl

-

from ambient up to 95C [15]. Potable water typically contains chloride concentrations of

10 to50 ppm [16]. If the chloride concentration of the plant water supply was maintained at these

levels,

it is unlikely Type 304L piping would suffer corrosion. However, the presence of the heat

tint has

compromised the corrosion resistance of the piping at the welds in this instance, leading to

significant localised pitting and eventual pipe leaking.

(a)


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(b)

Weld

Pipe ID surface


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3

PITTING CORROSION TESTING OF WELD SAMPLES FROM A WATER

RECYCLING

PLANT

3.1

Background of the testing

During the construction of a water recycling line in the same dairy plant, a portion of site-

welded

Type 316 pipe was inspected with radiography testing (approx. 80 welds in total). A

number of

locations were found to be oxidised due to insufficient purging of the ID surface.In total,a

near

continuous heat tint was observed on seven welds and isolated oxide film was found on

anothertwelve welds. In order to determine the quality and suitability of these welds, two welds were

submitted for pitting corrosion testing using ASTM G48 03 [17], to reveal locations

susceptible to

pitting corrosion.

The as received pipe samples are shown in Figure 7. Both samples were in the as-welded

condition without passivation. Samples were cut to expose the interior appearance of the

circumferential welds. Insufficient purging during welding had led to the development of a

heat tint

film with various colours ranging from dark blue to dark brown near the weld (Figure 7b). At

the

weld root, defects such as corrosion, lack of fusion and undercut were observed. A detailed

view of

these defects is seen in Figure 8a. Slag inclusions could be found in the weld metal in some

locations and a typical example from the large pipe is shown in Figure 8b.

Figure 7: Photographs showing the weld samples submitted for corrosion testing.

Figure 8: Photographs showing the weld defects observed from both the internal and external

surface of the pipe.

(a)

(b)

(a)(b)


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3.2 Experimental

A total of eight samples (four samples from each pipe) were removed for testing at 22C and

55C

(i.e.the typical temperature range that these pipes would be exposed to during service). Theedges

of each sample were ground with 120-grit abrasive paper as instructed in ASTM G 48 03.

Half of

the samples were tested in the as-received condition and the other half were tested after

carrying

outa passivation treatment as specified by the fabricator:

Passivation Procedure:

Pre-rinse the samples with water.

Make-up a 3% caustic solution and heat the solution to 60C. Rinse the samples with

the caustic solution till clear.Make-up a 4% nitric solution and heat the solution to 45C. Rinse the samples with the

acid solution for 60 minutes till clear.

Clean the samples with acetone in an ultrasonic bath before use.

It was observed that the heat tint was not removed after this passivation process.

The samples were exposed to a chloride-containing solution, which is expected to be similar

to the

condition within a pit or crevice site on ferrous alloys within a chloride-bearing environment

[14]. As

instructed in ASTM G 48 03, Method A, a 6% FeCl

3

(by mass) solution was made (i.e. 100g of

reagent grade ferric chloride, FeCl

3

H

2

O in 900 ml of Type IV reagent water). Sample strips were

then immersed in the above solution for up to three days at the test temperatures.The test

solutiontemperature was controlled by a water bath.

3.3 Results

The results after the exposure tests are summarised in Table 1. Weight loss was observed in

all

specimens, indicating pitting corrosion had occurred in all tests.

Table 1: Summary of pitting corrosion testing results.

Day 1Day 2

Day 3


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Pitting Depth

(mm)

Pitting Depth

(mm)

Pitting Depth

(mm)Sample ID

&

Condition

Weight change

(mg/cm

2

)

Max. Ave.

Weight change(mg/cm

2

)

Max.

Ave.

Weight change

(mg/cm

2

)

Max. Ave.

1 P

-0.4762 0.114

0.033 -0.747 1.072

0.310 -1.087 1.470

0.870

2 A -0.4627 0.416

0.144 -0.689 1.465

0.396 -1.090 1.465

0.996

3 P

-0.6505 0.9400.429 -1.150 1.480

1.041 -1.657 1.480

1.242

4 A

-0.75 0.993

0.846 -1.089

1.085

0.926 -1.659 1.486

1.131

5 P

-0.4298 0.0640.022 -0.829 0.641

0.181 -1.312 1.517


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0.610

6 A -0.407 0.941

0.046 -0.679

0.208

0.130 -1.043 0.658

0.5737 P

-0.7745 1.408

0.970 -1.476 1.497

1.254 -1.943 1.510

1.406

8 A

-0.76 1.136

0.761 -1.473

1.444

0.794 -2.064 1.520

1.049

: A - Passivated, P -As received

3.3.1 Ferric chloride pitting testing at 22C

For specimens exposed at 22C for 24 hours, specimens with the passivation treatment

exhibited

a better pitting corrosion resistance than the as-received specimens (Figure 9), i.e. less and

shallower pits were formed on the passivated specimens. Most of the pits were located in the

parent metal (PM) near to the heat tint patches.


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Figure 9: Photographs of (a) sample 5 (passivated) and (b) sample 6 (as-received) after oneday

exposure. The pits were concentrated near the heat tinted area as indicated (circled).

The severity of the pitting damage increased with increasing exposure time. In all tests, the

passivated specimens exhibited better pitting corrosion resistance than the as-received

specimens.

3.3.2 Ferric chloride pitting testing at 50C

The pitting damage,in terms of size and depth of pits, was found to be more severe at 50C

compared to the 22C exposure tests. As with the 22C exposure tests, most of the pits werefound in the PM near to the heat tint patches. Again, passivated samples exhibited better

pitting

corrosion resistance than samples inthe as-received condition.

3.4 Discussion

Both submitted pipes exhibited various weld defects such as corrosion, slag inclusions, lack

of

fusion and undercut at the weld root. These defects create ideal sites for pitting since the

diffusion

of oxygen is very limited in these sites. In long term service, debris is more likely to be

caught

along the weld. This will lead to the accumulation of debris which provides more favourable

sites

for pitting corrosion.

It was found that the current cleaning and passivation process was insufficient, i.e. the

heat tint

was not adequately removed under the current passivation process. During pitting corrosion

tests,

most of the pits in the PM were found to have initiated near the heat tint patches. This can beexplained by the crevice/pitting corrosion mechanism described in Section 1. It is widely

accepted

that the heat tint film on the stainless steel surface and the underlying chromium depleted

material

should be removed using a mechanical method or a pickling treatment before passivation. In

the

case of insufficient pickling, the subsequent passivation treatment will not be able to

build the

desired passive layer and pitting corrosion will occur in the area with heat tint.

As a result of the pitting corrosion testing and analysis, it was recommended that the currentdegreasing and passivation treatment be reviewed, and that a pickling treatment be included

prior


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to passivation.It was also recommended that the welding procedure and welding records be

reviewed to ensure appropriate weld quality is achieved.

4 CONCLUSIONS

The heat tint formed on stainless steel during welding, cutting and other fabricatingprocessescan

significantlyreduce theresistance to pitting and crevice corrosion. Fabricators and end users

should

be aware of the potential impact of heat tints on welded stainless steel components, and use

appropriate preparation, purging during welding, descaling/pickling and passivation

treatments.

Post weld cleaning of heat tint using either a mechanical method orpicklingtreatment removes

weld

scale and heat tint, while a passivation treatment restores the passive film for optimum

corrosionresistance. However, welding carried out on-site may not allow these practices due to safety

and

access issues. In this case, both the fabricator and end user should consider the exposure

environment and agree on an acceptable heat tint level and ways to achieve this.

(a)

(b)


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Although ASTM G 48 03, Method A is designed for evaluating pitting

corrosion resistance in

chloride-bearing environments, it can be used to evaluate the potential for pitting damage and

effectiveness of post-weld cleaning.

REFERENCES:

[1]. S. turner, F.P.A. Robinson 1989. Corrosion 49, 710 716.

[2]. K. Asami, K. Hashimoto 1979. Corros. Sci. 19, 1007 1017.

[3]. T. von Moltke, P.C. Pistorius, R.F. Sandenbergh 1992. First In. Chromium Steel &

alloys

Congress, INFACON 6, vol.2, Chromium Steel and Alloy 185 195.

[4]. P.K. Rastogi, B.K. Shah, A.K. Sinha, P.G. Kulkarni 1994. Brit. Corros. J. Vol. 29 78

80.

[5]. S. Azuma, H. Miyuki. J. Murayama. T. Kudo 1990. Corros. Eng. 667 676.

[6]. R.F.A. Pettersson, J. Flyg. 2005. Influence of weld oxides on the corrosion of 316Lstainless

steel. Eurocorr 2005, Paper 664.

[7]. G. Berz, G.K. Wehner, L. Toth, A. Joshi 1974. J. Appl. Phys. 45 5312 -5316.

[8]. WTIA, Chapter 4: Types of corrosion attack, WTIA/ACA technical note no. 13,

Stainless steels

for corrosive environments, 1998.

[9]. M. Keijzer 2004. Welding: to pickle or not pickle: no doubt about it. Published by: KCI

Media

B. V.

[10]. G.E. Coates 1990. Mater. Perform Vol. 29 61 -65.

[11]. A.H. Tuthill, R. Avery 1992. Adv. Mater. Process Vol. 142 34 -38.

[12]. American Welding Society Standards AWS D18.1/D18.1M:2009 .Specification for

Welding

of Austenitic Stainless Steel Tube and Pipe Systems in Sanitary (Hygienic) Applications.

[13]. American Welding Society Standards AWS D18.2:2009, Guide To

Weld Discoloration

Levels On Inside Of Austenitic Stainless Steel Tube.

[14]. J.Vagn Hansen. Reference colour charts for purity of purging gas in stainless steel

tubes,

Force Institute Report 94.34.

[15]. WTIA, Chapter 7: Stainless steel selection for specific environments, WTIA/ACAtechnical

note no. 13, Stainless steels for corrosive environments, 1998.

[16]. WTIA, Chapter 8: Corrosion in specific industries, WTIA/ACA technical note no. 13,

Stainless steels for corrosive environments, 1998.

[17]. ASTM G 48 03 Standard test methods for pitting and crevice corrosion resistance of

stainless steels and related alloys by use of ferric chloride solution

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A Case Study of Stainless Steel Water Supply Pipe Corrosion Caused by Weld Heat Tint