Long-term corrosion test of stainless steel for

water heater applications

Elisabeth Johansson, Sukanya Hägg Mameng

Avesta Research Centre, Outokumpu Stainless AB, Avesta/Sweden

Abstract

Stainless steels are used in domestic water heaters where their inherent corrosion resistance in potable water can be utilized without the use of lining or anode. In the case of duplex stainless steel their high strength can be utilized for a lighter design. It has been shown that proper design, welding procedures and post weld cleaning are important factors for achieving optimum corrosion resistance for stainless steel in water heater applications.

In this study long term test corrosion tests have been carried out for up to one year in water with 250 and 500 ppm chlorides at a temperature of 75°C. The tested materials include

ferritic, austenitic and duplex stainless steel grades. Results were evaluated with regard to pitting and crevice corrosion. Special emphasis was placed on the effect of weld oxide and different degrees of post weld cleaning, as well as different types of weld joints.

The long term test results are compared with short term electrochemical measurements such as pitting potential. Also experiences from stainless steel in domestic water heaters are discussed and compared with the laboratory corrosion results.

1. Introduction

Basically, a water heater contains a cylinder where water for domestic purposes is stored and heated. Heating can be electrical or via heat exchange from hot water heated by electrical power, solar power, gas or oil fired boilers. Properties required for all material used in water heating cylinders includes resistance to localized corrosion (pitting, crevice and stress corrosion cracking), good formability, weldability and high mechanical strength. As water heaters are pressure vessels, a higher mechanical strength can be utilized to reduce the wall thickness and thus the total weight of the cylinder.

Cylinders for domestic water heaters and boilers are traditionally produced in enameled carbon steel, copper, copper-clad carbon steel or stainless steel. With their inherit corrosion resistance to potable water, using stainless steel means that a water heater may not require sacrificial anodes and enables a considerable lighter design and longer lifetime compared to enameled cylinders. As a result, the use of stainless steels in domestic heater applications is steadily increasing. The main mode of corrosion for this kind of application is typically pitting corrosion, although if an unfavorable design with crevices is used crevice corrosion can occur.

One advantage of stainless steels is the multitude of grades which are available with their different combination of properties. Austenitic steels used for water heater cylinders typically contain 17-18% chromium, 8-10 % nickel and up to 2.1% molybdenum. The austenitic structure gives very good elongation and welding properties, and adding molybdenum improves corrosion resistance. However, the high nickel content has an appreciable effect on the price. With 11.5-18% chromium and sometimes also molybdenum (up to 2.1%), the ferritic grades are free from nickel, but with reduced elongation and welding properties. Ferritic grades without molybdenum addition have limited corrosion resistance. Duplex stainless steel grades with their microstructure of about 50% ferrite and 50% austenite utilizes the best of both worlds. Alloy contents of 21.5– 25% chromium, 1-7% nickel and up 4% molybdenum gives the duplex grades a wide range of corrosion resistance while still

retaining sufficient ductility. Addition of nitrogen adds strength and corrosion resistance as well as stabilizes the austenitic phase and improves welding properties. The fastest growing material group for water heaters in the last decade has been the high-strength lean and standard duplex grades. Figure 1 shows a schematic of some different stainless steel grades and their position with regard to corrosion and mechanical strength.

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LDX2101

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2205

LDX2404

4521

44044509/4301

Austenitic

Duplex

Ferritic

Figure 1: Schematic illustration of corrosion resistance (as PRE) and mechanical strength (as minimum proof strength for thin strip as per EN 10028-7) for some austenitic, duplex and ferritic stainless steel grades.

The corrosive environment that stainless steel encounters in a domestic water heater includes a water temperature of 60 to 90°C and a chloride content of up to 1000 ppm. The European Union drinking water directive limits the chloride content to 250 ppm [1], and the WHO Guidelines for drinking-water quality states that a higher content can give rise to detectable taste in water [2]. Nevertheless, chloride contents exceeding 250 ppm are not unusual in some parts of Europe and the rest of the world. Other factors such as water treatment by chlorination, hardness of the water and the presence of contaminants in the water will also affect its corrosivity towards stainless steel. Normally, the outer surface of the tank does not experience a particularly corrosive environment.

In this study long term corrosion tests were carried out for up to one year in water with 250 and 500 ppm chlorides at a temperature of 75°C. The tested materials include ferritic,

austenitic and duplex stainless steel grades. Results are evaluated with regard to pitting and crevice corrosion. Special emphasis was placed on the effect of weld oxide and post weld cleaning. Long term test results were compared with short term electrochemical measurements.

2. Experimental

2.1 Materials

The materials included in this study are presented in Table 1 with their typical chemical compositions. As shown, stainless steels with ferritic, austenitic and duplex microstructure are included and as made evident by their pitting resistance equivalent (PRE) value, they cover a wide-range of corrosion resistance. All materials were cold rolled with a thickness between 0.8 and 1.8 mm, which is representative for material used in water heater applications. All corrosion testing was carried out on the mill finish surface, as this is the surface that experience service in water heater cylinders.

Table 1: Tested materials

Stainless steel grade Typical chemical composition [wt%] Micro-structure

PRE* Outokumpu EN C N Cr Ni Mo Others

4307 1.4307 0.04 18.1 8.1 Austenitic 18

4404 1.4404 0.02 17.2 10.1 2.1 Austenitic 24

LDX 2101® 1.4162 0.03 0.22 21.5 1.5 0.3 5 Mn, Cu Duplex 26

2304 1.4362 0.02 0.10 23 4.8 0.3 Cu Duplex 26

LDX 2404® 1.4662 0.02 0.27 24 3.6 1.6 3 Mn, Cu Duplex 34

2205 1.4462 0.02 0.17 22 5.7 3.1 Duplex 35

4509 1.4509 0.02 18 Ti, Nb Ferritic 18

4521 1.4521 0.02 18 2 Ti, Nb Ferritic 25

* PRE = %Cr + 3.3%Mo+16%N

2.2 Preparation of welds

All the investigated stainless steel grades were autogenously TIG welded. Filler metal was deliberately avoided to give a more severe test as welding with filler would probably improve

the performance. Samples of 200300 mm size, were cut in half in the long direction, joint preparation was performed on the cut edges and the two pieces then welded back together in a butt joint. Two sets of welded samples were produced.

The welding parameters for the first set are shown in Table 2. These were butt welds with appropriate heat input and shielding and backing gases. Subsequent to welding samples were shot blasted and pickled until they appeared free from weld oxides. The pickling parameters are given in Table 3. Austenitic and duplex grades were pickled in a mixed acid (HNO3 + HF) bath. However, as the mixed acid is too aggressive for ferritic grades, these were pickled using pickling paste.

Table 2: Welding parameters for cleaned (pickled) welds

Grade Thickness

Joint type Heat input Shielding gas

(top) Backing gas

(root) [mm] [kJ/mm]

4301 1.3 Butt 0.21 Ar Ar

4404 1.25 Butt 0.18 Ar Ar

LDX 2101® 1.5 Butt 0.23 Ar + 2% N2 90% N2 + 10% H2

2304 1.5 Butt 0.24 Ar + 2% N2 90% N2 + 10% H2

LDX 2404® 1.8 Butt 0.34 Ar + 2% N2 90% N2 + 10% H2

2205 1.5 Butt 0.21 Ar + 2% N2 90% N2 + 10% H2

4521 0.8 Butt 0.17 Ar Ar

4509 1.1 Butt 0.18 Ar Ar

The second set of welds included the steel grades shown in Table 4. In contrast to the first set of pickled welds, some of these were welded without backing gas which results in a blue root oxide and some were welded with backing gas, but without subsequent removal of weld oxides. This gave a yellow oxide, which generally can be accepted. The appearance of pickled welds and welds with blue and yellow root oxide is shown in Figure 2. Some grades were also welded with a 20 mm overlap in order to simulate a crevice configuration with remaining oxides. Pickling was also performed on overlap joints, and in this case pickling paste (see Table 3) was used in order to pickle the outside surfaces while leaving any oxides in the crevice.

Table 3: Pickling parameters

Grade Joint type

Pickling medium Temperature Time

[°C] [min]

4301 Butt 3 M HNO3 + 3 M HF 53 3

Overlap BlueOne™ Pickling Paste 130 20-25 105

4404 Butt 3 M HNO3 + 3 M HF 53 3

LDX 2101® Butt 3 M HNO3 + 3 M HF 58 20

Overlap BlueOne™ Pickling Paste 130 20-25 390

2304 Butt 3 M HNO3 + 3 M HF 58 60

LDX 2404® Butt 3 M HNO3 + 3 M HF 58 150

2205 Butt 3 M HNO3 + 3 M HF 58 150

Overlap RedOne™ Pickling Paste 140 20-25 660

4521 Butt GreenOne™ Pickling Paste 120 20-25 120

4509 Butt GreenOne™ Pickling Paste 120 20-25 210

Table 4: Welding parameters for welds with remaining weld oxides

Grade Thickness

Joint type

Heat input

Shielding gas (top)

Backing gas (root)

Resulting root oxide

[mm] [kJ/mm]

4301 1.3 Overlap NA Ar Ar Yellow

LDX 2101® 1.5 Butt 0.23 Ar + 2% N2 90% N2 + 10% H2 Yellow

Butt NA Ar + 2% N2 None Blue

Overlap NA Ar + 2% N2 90% N2 + 10% H2 Yellow

2304 1.5 Butt 0.24 Ar + 2% N2 90% N2 + 10% H2 Yellow

Butt NA Ar + 2% N2 None Blue

2205 1.5 Butt 0.21 Ar + 2% N2 90% N2 + 10% H2 Yellow

Butt NA Ar + 2% N2 None Blue

Overlap NA Ar + 2% N2 90% N2 + 10% H2 Yellow

4521 0.8 Butt 0.17 Ar Ar Yellow

Butt NA Ar None Blue

Blue oxide

(without backing gas)

Yellow oxide

(with backing gas)

Clean

(with backing gas and pickled)

Figure 2: Appearance of root side of 2304 autogenous TIG welds.

2.3 Long-term immersion testing

Two types of specimens were used in this study, welded specimens and base material with crevice formers, both with a size of 60×30 mm and with the cut edges dry surface ground to 320 mesh. The welded specimens had the weld located centrally and parallel to the short edge while the crevice specimens had a hole Ø12mm drilled in the middle of the sample (Figure 3). All specimens were weighed before exposure. The drilled specimens were mounted on an insulated bolt of C-276 or Ti with crevice washers of INCO type (with 20 crevice sites per side) between each specimen, creating a rack. The rack was then tightened with a torque of 1.58 Nm.

The test specimens were exposed to two different environments, simulating waters that a domestic water heater is likely to come in contact with. The first environment was chosen according to EU and WHO drinking water directives i.e. a maximum chloride content of 250 ppm. The other environment was chosen to simulate somewhat more aggressive waters with a higher chloride content of 500 ppm. The test temperature was 75°C.

The test solution was aerated throughout the whole test period by purging air through the solution using a diaphragm pump. The entire test solution was changed once every month to remove possible corrosion products that could contaminate the solution.

The open circuit potentials for crevice specimens were recorded once every week using a platinum counter electrode and a saturated calomel reference electrode.

At the end of the test period, the specimens were removed and cleaned with soap and water

using a nylon bristle brush and then weighed and visually examined at 20 magnification. Crevice corrosion is considered to be present if the local attack underneath the crevice former is 0.025 mm or greater in depth.

Figure 4 shows the set-up for the long-term test. Pickled welds and base material with crevice formers were exposed for one year in 250 and 500 ppm chlorides. Welded specimens with remaining weld oxides and overlap joints were tested for six months in 250 ppm chlorides.

Figure 3: Example of test specimens used in this investigation.

Figure 4: Long term test set-up.

3. Results and discussion

In Table 5 results from one year testing in 250 and 500 ppm Cl- at 75°C are summarised.

At 250 ppm Cl- crevice corrosion was only found on the ferritic grade 4509 with a maximum depth of 0.069 mm. Pitting corrosion was found on cut edges on creviced samples of LDX 2101® and, but there was no weight loss. Pitting corrosion was also found on the rolled surface on one of the 4301 welded specimens, and the deepest pit was found to be 0.052 mm.

At 500 ppm Cl- corrosion attack was found on the welded 4404 specimens, where pitting was found on the rolled surfaces with maximum depth were 0.046 mm The pits on the rolled surfaces were located at the outer edge where the weld oxide had been present before pickling. Etching, defined as an attack with a depth less than 0.025 mm, was found on the creviced specimens of 4509, 4521 and 4301 while shallow pitting was found on one welded sample of 4404.

Results from the open circuit potentials for the creviced specimens are summarised in Figure 5 with average values as well as standard deviation from the weekly measurements. The potential for the different specimens typically varied between 50 and 350mVSCE. Lowest alloyed ferritic grade 4509 had the lowest potentials throughout the tests. Although there is overlapping, samples tested in 500 ppm chlorides had a slightly higher open circuit potential compared to those in 250 ppm chlorides.

It is seen that the results for the two environments are generally in good agreement, the exception being pitting of the welded 4301 specimen and crevice corrosion of 4509, which both occurred only in the milder environment (250ppm). This indicates that conditions are borderline for these grades and they should be used with caution in water heater applications.

Table 5: Summary of one year test at 75°C for crevice, and welded and pickled samples, with maximum depth of crevice corrosion and location of pitting.

Grade

250 ppm Cl-

500 ppm Cl-

Crevice samples Welded and

pickled samples Crevice samples

Welded and pickled samples

4301 NC PB, E C (< 0.025 mm) NC

4404 NC NC NC PB

LDX 2101® E NC NC NC

2304 NC NC NC NC

LDX 2404® NC NC NC NC

2205 NC NC NC NC

4509 C (0.069 mm) PW C (< 0.025 mm) C (< 0.025 mm)

4521 NC NC C (< 0.025 mm) C (< 0.025 mm)

No corrosion Corrosion Attack on cut edge / etching (<0.025 mm]

NC= no corrosion, C = crevice corrosion, PB = pitting corrosion in base material, PW = pitting corrosion in weld, E = edge attack

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Figure 5: Average OCP measured on crevice samples during one year of exposure. Error bars indicate the standard deviation.

No localised corrosion attack could be noticed on any on the specimens after the second series of tests, carried out for six months in 250 ppm chlorides at 75°C. Oxide dissolution and discolouration was noticed on almost all samples where weld oxide was still present. The solution developed a brown colouration, which was very pronounced just before the solution was changed and, which indicated that further oxidation of the remaining weld oxides had occurred. No significant difference could be noticed between samples welded with and without backing gas, i.e. yellow and blue root oxide.

LDX 2101® and 4301 both suffered pitting, both of the pickled and un-pickled overlap joints in after six months of testing in 250 ppm. The samples with remaining weld oxides had more severe corrosion attack in terms of propagation. The overlap joints were cut in half to investigate the presence of crevice corrosion. The cross sections were polished using SiO2

slurry and examined using a light optical microscope. Minor crevice corrosion/etchings was observed for 4301, while 2205 showed no signs of crevice corrosion, see Figure 6. When testing using INCO crevice washers with torque in 500 ppm chlorides for one year, 4301 showed signs of crevice corrosion after one year exposure (see Table 5). However, the two types of crevices tested are quite different, the second one being more controlled and reproducible. The crevices differ also in terms of material (metal-metal vs. metal-plastic), “tightness”/width, presence of a welded microstructure and presence of oxides. The overlap joint in this case showed worse performance which highlights the detrimental effect of weld oxide in combination with a crevice.

Table 6: Summary of six month test in 250 ppm Cl- for samples with different levels of weld oxides and

overlap joints

Sample type Root oxide Grade Oxide dissolution Corrosion attack

Butt weld, pickled with mixed acid

None LDX 2101® NC

2304 NC

2205

NC

4521 NC Butt weld, as-welded without backing gas

Blue LDX 2101® Yes NC

2304 Yes NC

2205

Yes NC

4521 Yes NC Butt weld, as-welded with backing gas

Yellow LDX 2101® Yes NC

2304 Yes NC

2205 Yes NC

4521 Yes NC

Overlap joint, pickled with pickling paste

None 4301 C, PW, PB

LDX 2101® C, PW

2205 NC

Overlap joint, as-welded with backing gas

Yellow 4301 Yes C, PW

LDX 2101® Yes C, PW

2205 Yes PW

No corrosion Corrosion

NC= no corrosion, C = crevice corrosion, PB = pitting corrosion in base material, PW = pitting corrosion in weld, E = edge attack

4301 LDX 2101® 2205

Figure 6: Cross sections of overlap joints in 4301, LDX 2101

® and 2205 with remaining oxides at 20

and 80 magnification.

The results for some steel grades in this study can be compared to previously published engineering diagrams for stainless steels as shown in Figure 7 [3]. These diagrams offer a guideline for materials selection in different waters under certain conditions and are valid for natural pH, naturally oxygen saturated and slightly chlorinated water (<1 mg/l Cl2). If the environments tested in this study are marked out in the engineering diagrams, see Figure 7, one would expect that some grades would suffer corrosion. The environment of 250 ppm chlorides and a temperature of 75°C, is above the line for crevice corrosion for grades 4301 and 4404 and above the pitting line for 4301 whereas it is borderline for 4404. One would also expect some corrosion on 4509, 4521 and LDX 2101® due to similar and/or lower PRE values than 4301 and 4404. In the environment containing 500 ppm chlorides there is also a risk for crevice corrosion on 2205, while 4404 and grades with lower PRE would be expected to suffer from pitting corrosion. This predicts more cases of corrosion than were seen in the one year test but the difference can be explained by the absence of chlorination in the tests carried out in this study. A study on the dependence on chlorine dosage for corrosion of stainless steel showed that the redox potential was typically 500 mVSCE for 0.5 ppm chlorine and 700 mVSCE for 1 ppm chlorine, i.e. much higher than the maximum of 350 mVSCE reached in this study [4]. Moreover, crevice corrosion was observed on 4404 at 50°C at 200 ppm Cl-

with 0.5 ppm chlorine.

Figure 7: Engineering diagrams for stainless steels [4]. Red triangles mark the environments tested in this study.

The results can also be compared to studies that have been presented on the limiting conditions of pitting corrosion [5,6]. These studies used potentiodynamic polarization curves for defining engineering diagrams for different potential levels. As the maximum OCP values measured in this study was 300-400 mVSCE, these engineering diagrams for 300 and 400 mVSCE are compared to the test conditions in the current study in Figure 8. The comparison shows that 500 ppm Cl- and 75°C is close to the limit for 400 mVSCE for grades 4301 and LDX 2101®. These studies clearly underline the importance of the potential for the

pitting corrosion resistance for stainless steel.

Also, a 30 day exposure in a non-chlorinated system at 80°C showed that a chloride content >2000 ppm Cl- is necessary in order for pitting corrosion to occur [6]. The open circuit potential was measured to 200-250 mVSCE which is somewhat lower that what was measured for crevice samples in this study.

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Figure 8: Comparison of test conditions (red marks) with limiting conditions at 300 and 400 mVSCE for short term electrochemical tests for grades 4301, 4404 and LDX 2101

® [5,6].

Figure 10: a) Crevice between end cap and cylinder and (b) corrosion products running from the crevice on the inside [7].

Figure 9: Pitting attack in weld oxide formed on the (a) topside at connection and (b) root side of a cylinder [7].

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Previously published work addressed experience and recommendations on welding of stainless steel for water heater applications [7]. The negative effect of weld oxides on the corrosion resistance of stainless steel was illustrated and it was concluded that crevices should be avoided and residual weld oxide removed by pickling. Examples of corrosion initiating at non-pickled welds and crevices are shown in Figures 9 and 10.

Considering the results in this study, corrosion associated with welds and remaining weld oxides is most probably also a result of harsher condition than was tested here. Examples of factor that may increase the risk for corrosion of stainless steel in water heater applications include:

Chlorination which increases the redox potential significantly.

Formation of lime scale where crevice corrosion can initiate if the water is hard.

Stray currents which can increase the potential.

Poor design, e.g. with severe crevices.

Carless use of soldering flux containing halides.

Failure to remove weld oxides.

4. Conclusions

In conclusion, this study with one year of exposure shows that corrosion is unlikely on all of the tested stainless steel grades in 250 ppm Cl- at 75°C, except for grade 4509, unless the potential is increased e.g. by chlorination. Even at 500 ppm Cl- all tested duplex grades are free from corrosion attack. Remaining weld oxide does not initiate corrosion within six months, although oxide dissolution is observed. However, a tight crevice has an adverse effect on the corrosion resistance resulting in pitting corrosion during the same time period.

The results from one year exposure match up well with short term electrochemical tests performed at potentials of 300-400 mVSCE. Corrosion occurring on water heaters, also with remaining weld oxides, is probably a result of more severe operating conditions, e.g. with higher levels of chlorination.

A number stainless steel grades, including austenitic, ferritic and duplex stainless grades, are suitable for water heater applications. Selection of an appropriate grade should be based on the operating conditions and its corrosivity. In areas with harsh conditions, e.g. with high chloride content in combination with chlorination, a higher alloyed grade such as duplex 2304 or 2205 can be necessary. Under less aggressive conditions lower alloyed grades such as lean duplex LDX 2101®, austenitic 4404 or ferritic 4521 can provide adequate corrosion resistance for water heater cylinders.

5. References

[1] Council Directive 98/83/EC of 3 November 1998 on the quality of water intended for human consumption [1998] OJ L330/32.

[2] Guidelines for drinking-water quality, fourth edition, World Health Organization 2011.

[3] Corrosion Handbook, tenth edition, Outokumpu, 2009.

[4] S. Mameng, R. Pettersson, “Localised corrosion of stainless steel depending on chlorine dosage in chlorinated water”, EUROCORR 2011, Stockholm, Sweden, 2011.

[5] S.H. Mameng, R. Pettersson, “Limiting conditions of pitting corrosion of stainless steel EN 1.4404 (316L) in terms of temperature, potential and chloride concentration”, EUROCORR 2013, Estoril, Portugal, 2013.

[6] S.H. Mameng, R. Pettersson, “Limiting conditions of pitting corrosion for lean duplex stainless steel as a substitute for standard austenitic grades”, EUROCORR 2014, Pisa, Italy, 2014.

[7] E.M. Westin & D. Serrander, “Experience in welding stainless steels for water heater applications”. Welding in the World I56 (2012) 5/6 14 – 28. IIW Doc.-No. IX-2357-11.