corrosion testing of several waterwall coatings history... · corrosion testing of several...

Corrosion Testing of Several Waterwall Coatings

Prepared for Dominion Power

April 15, 2013

Prepared by John N. DuPont, PhD

Energy Research Center

Lehigh University

Bethlehem, PA 18015

Objective:

The objective of this project was to determine the corrosion resistance of three different

Emisshield coatings (M1, M2, M6) relative to bare steel and a commercially available coating

(45CT).

Testing Procedure

The samples were provided in the ready-to-test condition by Dominion Power. All samples were

½” x ½” x ¼” and were coated on all sides. The samples were corrosion tested at 600 oC in a

tube furnace using a simulated combustion gas of the following composition (by volume): N2-

10%CO-5%CO2-2%H2O-0.12%H2S. A gas flow rate of 250 cc/min was used, and two samples

of each coating and bare steel were placed into the furnace at the start of the test. Weight gain

and post-test examinations by light optical microscopy (LOM) were used as indicators of

corrosion resistance. Each set of samples was removed after the first 250 hours of testing and

weighed. One sample set was then returned to the furnace for an additional 250 hours of

exposure. Upon completion of the second 250 hour exposure, the second set of samples was

removed and weighed. Each sample was placed in a separate crucible during the tests. The

crucible and sample were weighed before and after testing to measure the weight gain of the

sample associated with each exposure time. The crucibles were used to collect any corrosion

scale that spalled from the sample during the test.

The specimens were mounted in epoxy directly after testing to maintain the integrity of the

corrosive scales and subsequently cross-sectioned with a high speed abrasive saw. Ethanol was

used as the lubricant during the cutting process to preserve the corrosion scale. The samples were

then remounted to produce full-size metallographic samples for microstructural observation by

LOM. Metallographic preparation was performed using an ATM Saphir automated grinding and

polishing machine. Grinding was performed using 320, 400, 600, 800 and 1200 grit SiC

metallographic paper for a total of approximately 2 minutes at each grit size. A saturated solution

of paraffin wax in kerosene was used in place of water as a lubricant during all grinding steps to

prevent any water soluble corrosion products from dissolving. Polishing to 1µm and 0.25µm was

done by hand using an aerosol diamond spray as the polishing medium in combination with an

alcohol based lubricant.

Additional specimens (M1, M6, and uncoated) in the as-received condition were prepared for

microhardness testing and microstructural investigation. The samples were mounted in bakelite

and ground and polished to a 0.05µm finish using standard metallographic procedures. Five

1000g Vickers hardness indents were made in each sample at randomly selected locations.

Hardness measurements were performed using image analysis software. After microhardness

testing, the samples were etched in 2% picric acid in ethanol for 20 seconds followed by etching

in 2% nitric acid in ethanol for 30 seconds to reveal the microstructure of the steel substrate.

Results

Figure 1 shows photographs of each sample prior to testing. Figure 2 shows the weight gain of

each sample after the first 250 hour exposure. There is very good agreement between the same

types of samples. At this point, the M1 and M6 coatings showed negligible (undetectable) weight

gain, while the M2 and bare sample exhibited very high weight gains indicative of extensive

corrosion. (The minimum detectable weight change with the scale used for these measurements

was 0.2 mg.) The thermal spray coating exhibited a small amount of weight gain. Photographs of

the samples after the first 250 hour exposure are provided in Figures 3 and 4.

Figure 5 shows the weight gain results for the first and second 250 hour exposures. The trends

observed after 500 hours are consistent with those that were observed after 250 hours. The

uncoated and M2 samples have continued to exhibit significant weight gain. There is only

modest weight gain on the thermal spray sample, and the M1 and M6 samples again showed no

detectable weight gain. Photographs of the samples after the 500 hour exposure are provided in

Figures 6 and 7.

Figures 8 through 10 show LOM photomicrographs of the samples (in cross-sectional view) at

various magnifications after the first 250 hour exposure. Similar LOM photomicrographs are

shown for the samples after 500 hours of exposure in Figures 11 through 13. These results are

consistent with the weight gain measurements. Note that the corrosion scale on the uncoated

sample is relatively thick and is similar to that observed on sample M2. Also not that both

corrosion scales have separated from the underlying steel sample, indicating poor adherence.

Based on results of previous tests conducted at Lehigh, these scales appear to be iron sulfides,

which are known to provide negligible protection. The thermal spray coating exhibits a modest

amount of corrosion product on the surface that is most evident at the highest magnifications.

The M1 and M6 coatings may exhibit a minor amount of corrosion product that is barely visible

at higher magnifications. An example of this is shown in Figure 14. The size of this scale,

together with the slight edge rounding that occurred at the surface during sample preparation,

make it difficult to identify this as a corrosion scale for certainty. Scanning electron microscopy

coupled with energy dispersive spectroscopy would be needed to clarify this.

There were occasional locations of the M1 and M6 coating that exhibited porosity. The presence

of porosity may be a concern because these coatings are thin relative to other coating types.

Examples of this are shown in Figure 15, where pores are shown that occupy a large fraction of

the coating thickness. These were almost always observed at sample corners. An isolated region

was also observed where the M2 coating was breached and therefore exhibited localized

corrosion at the breach (Figure 16). Lastly, There were areas within the thermal spray coating

that appeared to exhibit localized corrosion along the substrate/coating interface (Figure 17).

This is interfacial corrosion is attributed to rapid diffusion of the corrosive gas through splat

boundaries and porosity. Final coating failure often then occurs via failure along the corroded

interface.

Figures 18 and 19 show LOM photomicrographs of the steel substrate for the uncoated samples

and the M1 and M6 coated samples. The substrate for each sample consists primarily of ferrite

with a small amount of pearlite, which is typical of a low carbon steel. Abnormal grain growth

was observed near the edge of each substrate. The cause(es) of this are not known from this

work. Since the grain growth is also observed on the uncoated sample, it cannot be attributed to

the coating process. Table 1 shows the hardness results. The uncoated and M6 coated sample

substrates have very similar hardness values, while the substrate from the M1 coated sample is

slightly harder. In general, the coating process did not appear to have any significant effect on

the substrate microstructure or hardness.

Conclusion

Based on these results, coatings M1 and M6 appear to provide very good corrosion resistance in

the atmosphere tested. The performance is slightly better than the 45CT thermal spray coating.

Coating M2 provided negligible protection. Attention should be paid to possible defects in

coatings M1 and M6, since the samples are thin relative to other commonly used protective

coatings. The coating process did not appear to have any significant effect on the substrate

microstructure or hardness.

Figure 1: Corrosion coupons before testing

M2

Thermal

Spray

M1

M6

Uncoated

M1

M6

Uncoated

M2

Thermal

Spray

Figure 2: Coating corrosion performance after 250 hours

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

M1 M2 M6 Thermal Spray

Uncoated

We

igh

t G

ain

(g)

Coating Performance

250hrs

Figure 3: Corrosion coupons after 250 hour exposure

M2

Thermal

Spray

M1

M6

Uncoated

M1

M6

Uncoated

M2

Thermal

Spray


M2 Thermal

Spray M1

M6 Uncoated M1

M6 Uncoated

M2 Thermal

Spray

Figure 5: Coating corrosion performance after 500 hours

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

M1 M2 M6 Thermal Spray

Uncoated

We

igh

t G

ain

(g)

Coating Performance

250hr Avg 500hrs


M2

Thermal

Spray

M1

M6

Uncoated


M1 Uncoated M6

Thermal

Spray M2

M1 M2 M6

Thermal Spray Uncoated

Figure 8: Light optical micrographs of the

corrosion scale developed on coating surfaces

after 250hr exposure.

M1 M2 M6





Figure 14: Possible signs of corrosion scale buildup on coating M1

Figure 15: Defects in coatings (A) M1 and (b) M6 at sample corners

Figure 16: Breach in coating M2

Figure 17: Development of corrosion product at coating-substrate interface due to penetration of test gas

through thermal spray coating

Table 1: 1000g Vicker microhardness of selected specimens

Uncoated

M1

M6

Field 1 132.55

Field 1 151.92

Field 1 131.06

Field 2 133.46

Field 2 150.45

Field 2 131.35

Field 3 135.3

Field 3 153.03

Field 3 131.65

Field 4 131.95

Field 4 148.64

Field 4 130.76

Field 5 140.75

Field 5 148.64

Field 5 133.46

Average 134.8 ± 3.6

Average 150.5 ± 2.0

Average 131.7 ± 1.1

Uncoated M1 M6

Figure 18: Light optical micrographs of substrate

Uncoated M1 M6

Figure 19: Light optical micrographs of substrate edge

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