Report on Salt-Spray Testing based on...

STNP Subcommittee Task-Force on Corrosion Effects on Subsurface Transformers

Report on Salt-Spray Testing based on C57.12.32-2019

Prepared by Will Elliott & Thomas Dauzat – Prolec GE – Shreveport, LA

Submitted on 10/14/2020 – (Revised 10/22/2020)

Summary of Test Methods Defined in Submersible Equipment Enclosure Integrity Standard:

IEEE C57.12.32-2019 §4 lists the substrate test requirements, where the substrate is defined as the

material of which the equipment enclosure is constructed.

The control test panel materials are listed in §4.1 of the standard:

• AISI 409 stainless steel for equipment “intended for application in normal submersible

environments (such as not frequently or continuously submerged in salt water)”

• AISI 304L stainless steel for equipment “intended for application in especially severe

environments (such as frequent or continuous submersion in salt-water)”

The test requirements and evaluation criteria for testing any other enclosure material (not 409 or

304L) is listed in §4.3.2. For the purposes of this report, only metallic substrates were considered.

The thicknesses of the test panels were based on the required tank wall thickness per IEEE standards

C57.12.24-2016 §7.5.1.3 and C57.12.40-2017 §5.2.1:

• Test Requirements:

o 1500 hour salt-spray testing per ASTM B117 with the following substrate test panel

materials and quantities:

1. Quantity (5) bare stainless-steel control test panels (409 or 304L, depending on

intended service environment)

2. Quantity (5) bare substrate test panels (of any other enclosure material that is

proposed, except those materials listed as exempt in §4.3.2)

o Test Panel Requirements:

1. Stainless-steel control test panels per Figure 2:

• 3 inches wide X 4 inches tall

• 0.25 inches thick

• No weld bead required

2. Proposed substrate test panels per Figure 1:

• 3 inches wide X 4 inches tall

• 0.31 inches thick

• Weld bead is required

• Test Evaluation Criteria:

o A substrate test panel passes the test when both of the following criteria are met:

1. The average mass of the five proposed substrate test panels is not reduced from

corrosion by more than 2.5%, as calculated by Equation (1).

2. The average mass loss of the five proposed substrate test panels is not greater

than 5 times the average mass loss of the five control test panels, as calculated

by Equations (2) and (3).

• Formulas for Evaluation Criteria:

o Equation for evaluating the average mass loss for the proposed substrate test panels:

𝑚∆𝑠 =1

5∑

𝑚𝑠𝑛𝑖 − 𝑚𝑠𝑛𝑓

𝑚𝑠𝑛𝑖

5

𝑛=1

≤ 2.5% = 0.025 (1)

Where:

𝑚∆𝑠 is the average mass loss (percentage) for the quantity five proposed substrate

test panels tested

𝑛 is the sample-number assigned to the substrate test panel being evaluated, where

𝑛 = {1,2,3,4 𝑜𝑟 5}

𝑚𝑠𝑛𝑖 is the initial mass before testing (grams) of the substrate test panel sample-

number being evaluated

𝑚𝑠𝑛𝑓 is the final mass after testing (grams) of the substrate test panel sample-number

being evaluated

o Equations for evaluating the average mass loss of the proposed substrate test panels to

the average mass loss of the control test panels:

𝑚∆𝑐 =1

5∑

𝑚𝑐𝑛𝑖 − 𝑚𝑐𝑛𝑓

𝑚𝑐𝑛𝑖

5

𝑛=1

(2)

𝑚∆𝑠

𝑚∆𝑐≤ 5 (3)

Where:

𝑚∆𝑐 is the average mass loss (percentage) for the quantity five control test panels

tested

𝑛 is the sample-number assigned to the control test panel being evaluated, where

𝑛 = {1,2,3,4 𝑜𝑟 5}

𝑚𝑐𝑛𝑖 is the initial mass before testing (grams) of the control test panel sample-number

being evaluated

𝑚𝑐𝑛𝑓 is the final mass after testing (grams) of the control test panel sample-number

being evaluated

Summary of Modifications & Deviations from C57.12.32-2015 Test Requirements:

1. The testing performed only measured quantity (1) test panel samples per material.

▪ Due to this deviation, Equations (1) & (2) were modified for the purposes of this report,

since no average was necessary. Equations (4) & (5) were used for analysis; where 𝑛 =

1, as defined previously in the original equations:

𝑚∆𝑠 =𝑚𝑠1𝑖 − 𝑚𝑠1𝑓

𝑚𝑠1𝑖≤ 2.5% = 0.025 (4)

𝑚∆𝑐 =𝑚𝑐1𝑖 − 𝑚𝑐1𝑓

𝑚𝑐1𝑖 (5)

2. Painted test-panel samples were also evaluated for comparison purposes.

3. Weld beads were not used on either any test panel samples on the 1032 and 1500 hour tests.

Weld beads were used on all test panel samples (including control test panels) for the 3000

hour test.

4. Different dimensional test panel samples were tested (3 inch X 4 inch & 3 inch X 6 inch) to

evaluate the difference in test panel dimensions used in substrate testing versus coating

system testing.

5. Different test durations were performed:

▪ The 1032 hour test was performed to provide preliminary data for the Fall 2019 task-

force meeting.

▪ The 1500 hour test was performed to reflect the test duration in C57.12.32-2019.

▪ The 3000 hour test was performed based on feedback and a motion which passed in

the task-force in the Fall 2019 meeting.

Data Quality

Several test panel samples measured a negligible mass increase after salt-spray testing, between

0.1g and 0.3g. This mass increase is presumed to be a measurement or cleaning error (e.g. salt

intrusion that could not be removed due to the cleaning process), and was not considered in the

mass loss calculations, but is shown in the raw mass measurement data for clarity.

Summary of Testing Performed:

Description of Method:

1. Weigh the initial mass of all test panels

2. Expose the test panels for the duration (hours) specified in a salt-spray chamber per ASTM

B117-18 & B117-19

3. Rinse the test panels with de-ionized water, scour with a nonmetallic abrasive pad, rinse

again, and let dry

4. Weigh the final mass of all test panels

Test Durations Performed:

1. 1032 Hour Test (report dated 10/16/2019) per ASTM B117-18



All tests performed by AkzoNobel Coatings Inc Accelerated Weathering / Physical Testing Laboratory

Tables 1, 2 & 3 summarizes all test data, subsequent pages show the original test reports. The

tables list 0.0* for the weight change (ΔMass [g] column) for painted test panels which measured a

weight increase, which is believed to be a result of salt intrusion that was not removed by cleaning.

The tables list the test panel materials (304L, 409 & “Cu-Bearing”). The term “Cu-Bearing” was used

as a short-hand description for “copper-bearing steel” used in the IEEE submersible equipment

product standards, known more precisely as low-alloy carbon steel with copper content ≥0.2%.

All samples listed “PAINTED” were painted the same, with a zinc epoxy primer & a black epoxy top-

coat. Painted samples are not required per C57.12.32-2019 but were tested for reference.

Salt-Spray Test Data:

Table 1: 1032 Hour Salt-Spray Test Data

Test Panel Description

Test Panel Material

Test Panel Dimensions

Test Panel Paint

1032 Hour Test

Initial Mass [g]

Final Mass [g]

ΔMass [g]

ΔMass [%]

3"x4"x0.250" 304L BARE (control)

304L

3"x4"

Bare

380.3 380.4 0.0* 0.00%

3"x4"x0.250" 409 BARE (control)

409 379.8 379.5 0.3 0.08%

3"x4"x0.312" Cu-Bearing BARE

Cu-Bearing 464.2 448.8 15.4 3.32%

3"x4"x0.250" 304L PAINTED

304L

Painted

382.4 382.4 0.0 0.00%

3"x4"x0.250" 409 PAINTED

409 386.6 386.7 0.0* 0.00%

3"x4"x0.312" Cu-Bearing PAINTED

Cu-Bearing 469.0 469.0 0.0 0.00%

3"x6"x0.250" 304L BARE

304L

3"x6"

Bare

575.6 575.6 0.0 0.00%

3"x6"x0.250" 409 BARE

409 573.5 573.3 0.2 0.03%


Cu-Bearing 697.0 667.2 29.8 4.28%


304L

Painted

586.6 587.0 0.0* 0.00%

3"x6"x0.250" 409 PAINTED

409 583.5 583.5 0.0 0.00%


Cu-Bearing 709.6 709.9 0.0* 0.00%

NOTES:

1. ΔMass [g] = 0.0* indicates a weight gain was measured

2. 1032 Hour test panels did not include a weld bead



Test Panel Material


Test Panel Paint

1500 Hour Test

Initial Mass [g]

Final Mass [g]

ΔMass [g]

ΔMass [%]

3"x4"x0.250" 304L BARE (control)

304L

3"x4"

Bare

380.3 380.3 0.0 0.00%

3"x4"x0.250" 409 BARE (control)

409 377.7 377.3 0.4 0.11%


Cu-Bearing 462.3 438.8 23.5 5.08%


304L

Painted

384.1 384.1 0.0 0.00%

3"x4"x0.250" 409 PAINTED

409 387.4 387.5 0.0* 0.00%


Cu-Bearing 469.3 469.2 0.1 0.02%

3"x6"x0.250" 304L BARE

304L

3"x6"

Bare

577.1 576.9 0.2 0.03%

3"x6"x0.250" 409 BARE

409 572.5 572.0 0.5 0.09%


Cu-Bearing 697.9 661.2 36.7 5.26%


304L

Painted

586.0 586.1 0.0* 0.00%

3"x6"x0.250" 409 PAINTED

409 585.9 585.9 0.0 0.00%


Cu-Bearing 705.1 704.7 0.4 0.06%

NOTES:


2. 1500 Hour test panels did not include a weld bead



Test Panel Material


Test Panel Paint

3000 Hour Test

Initial Mass [g]

Final Mass [g]

ΔMass [g]

ΔMass [%]

3"x4"x0.250" +weld 304L BARE (control)

304L

3"x4"

Bare

390.0 390.0 0.0 0.00%

3"x4"x0.250" +weld 409 BARE (control)

409 396.6 395.8 0.8 0.20%

3"x4"x0.312" +weld Cu-Bearing BARE

Cu-Bearing 476.4 438.1 38.3 8.04%

3"x4"x0.250" +weld 304L PAINTED

304L

Painted

395.0 395.0 0.0 0.00%

3"x4"x0.250" +weld 409 PAINTED

409 400.0 400.0 0.0 0.00%

3"x4"x0.312" +weld Cu-Bearing PAINTED

Cu-Bearing 486.4 486.3 0.1 0.02%

3"x6"x0.250" +weld 304L BARE

304L

3"x6"

Bare

589.7 589.7 0.0 0.00%

3"x6"x0.250" +weld 409 BARE

409 586.6 585.9 0.7 0.12%

3"x6"x0.312" +weld Cu-Bearing BARE

Cu-Bearing 710.2 653.9 56.3 7.93%

3"x6"x0.250" +weld 304L PAINTED

304L

Painted

595.6 595.5 0.1 0.02%

3"x6"x0.250" +weld 409 PAINTED

409 596.6 596.4 0.2 0.03%

3"x6"x0.312" +weld Cu-Bearing PAINTED

Cu-Bearing 720.1 720.1 0.0 0.00%

NOTES:


2. 3000 Hour test panels did include a weld bead:

a. 304L test panels had a 308L weld bead

b. 409 test panels had a 309LSi weld bead

c. Cu-Bearing test panels had a MC-706 weld bead

Graphs of Salt-Spray Data:

Figure 1: Bare Metal Salt-Spray Mass Loss vs Test Duration

0.00%

0.25%

0.50%

0.75%

1.00%

1.25%

1.50%

1.75%

2.00%

2.25%

2.50%

2.75%

3.00%

3.25%

3.50%

3.75%

4.00%

4.25%

4.50%

4.75%

5.00%

5.25%

5.50%

5.75%

6.00%

6.25%

6.50%

6.75%

7.00%

7.25%

7.50%

7.75%

8.00%

8.25%

8.50%

1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000

Wei

ght

Loss

[%

]

Salt-Spray Test Duration [Hours]

Bare Metal Salt-Spray Test Data

3"x4"x0.25" 304L BARE 3"x4"x0.25" 409L BARE 3"x4"x0.312" Cu-Bearing BARE

3"x6"x0.25" 304L BARE 3"x6"x0.25" 409L BARE 3"x6"x0.312" Cu-Bearing BARE

Figure 2: Painted Metal Salt-Spray Mass Loss vs Test Duration

Comments on graphed test data:

• Figures 1 & 2 reflect the data listed in Tables 1 through 3 and graph the test panel mass loss

as a function of salt-spray test duration (1032, 1500 and 3000 hour).

o Note that the y-axis (weight loss) of the two graphs are not the same scale.

o Note that only the 3000 hour test panels included welds.

• It is critical to recognize that each datapoint on these graphs represents a different test panel,

and not the same test panel over time.

o Weight loss measurements require removal of corrosion products. Corrosion products

(oxide layers) inhibit corrosion. Therefore, cleaning a test panel and then subjecting it

to further testing would not be a true reflection of a test panels performance as a

function of time

• The value of these graphs would be higher if a larger quantity of test panels for each material

were measured.

o Note that some corrosion was observed on painted 304L but not bare 304L on one test

panel tested; whereas, one copper-bearing steel test panel did not measure corrosion.

o It is expected that with a larger number of painted test panel samples that the effect of

paint defects (see diverging data in Figure 2) will become more statistically relevant.

0.00%

0.01%

0.02%

0.03%

0.04%

0.05%

0.06%

0.07%

0.08%

0.09%

0.10%

0.11%

0.12%

0.13%

0.14%

0.15%

1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000

Wei

ght

Loss

[%

]

Salt-Spray Test Duration [Hours]

Painted Metal Salt-Spray Test Data

3"x4"x0.25" 304L PAINTED 3"x4"x0.25" 409L PAINTED 3"x4"x0.312" Cu-Bearing PAINTED

3"x6"x0.25" 304L PAINTED 3"x6"x0.25" 409L PAINTED 3"x6"x0.312" Cu-Bearing PAINTED

Evaluation of Test Data Compared to C57.12.32-2019:

This testing was performed to provide preliminary data to the task-force on the performance of

“copper-bearing steel” (low-alloy carbon steel with copper content ≥0.2%) per the C57.12.32-2019

standard, because it is a well-established enclosure material used for submersible equipment

It is important to note that due to the reduced number of samples tested (and the other deviations

noted) that these results are not definitive, but it is obvious that per C57.12.32-2019 that copper-

bearing steel will not pass the evaluation criteria defined in the standard. The first three rows of data

in Table 2 show the most relevant data for evaluation (1500 hour salt-spray data). Using the

formulas previously defined in this report, the following results were calculated:

1. The mass loss for copper-bearing steel was 5.08%.

2. The mass loss for copper-bearing steel was 46.2 times larger versus 409.

3. The mass loss for copper-bearing steel could not be evaluated versus 304L as noted below.

In principle it was not possible to evaluate the relative mass loss for copper-bearing steel in “severe

environments” because the 304L control test panel did not measure a detectable mass loss, however

it obviously corroded more than 5 times versus 304L. Nonetheless, it is presumed that with a larger

sample size (e.g. 5 samples as specified in the standard) that the mass loss may become non-zero.

Remarks on Test Data and Test Methods:

1. The different test panel dimensions for substrate testing vs coating testing did not

substantially affect the results.

2. The standard requires that the stainless-steel control test panel not be welded but requires

any proposed substrate test panel be welded. This is not an equal comparison in principle.

3. Salt-spray testing per B117 is typically done with multiple test panels in a single chamber. It is

believed that skilled test labs appropriately separate test panels of different materials to

prevent galvanic interaction between different material test panels. It should be noted that if

not accounted for, that effect could result in accelerated corrosion for carbon steel alloys

because it would act as a sacrificial anode, while protecting stainless-steel, and skew data.

4. A 0.0001 gram measurement resolution for mass measurements may be overly precise and

may result in unnecessary costs and limited lab availability to perform the tests.

5. Does the salt-spray test realistically represent the service environment, which may contain

more cathodic materials such as copper alloys?

6. Is comparing bare stainless-steel to bare copper-bearing steel a reasonable comparison? It

should not be ignored that many utilities in “normal environments” have successfully used

painted copper-bearing steel for many decades.

• Painted copper-bearing performed well, reducing corrosion 99.6% on average versus

bare test panels on the 3000 hour test. Paint defects do exist, but lower cost risk

mitigation options exist.

• A larger number of painted test panels would be required to determine the statistical

relevance, but 3000 hour stainless-steel test panels did exhibit corrosion when painted.

• The substantial capital investment impact of stainless-steel tanks is a valid concern for

utilities and may not result in a valuable increase in service life for many.

References:

1. IEEE C57.12.32-2019, IEEE Standard for Submersible Equipment—Enclosure Integrity

• See §4 for substrate test requirements

• See §4.1 for control test panel materials

• See §4.3.2 test requirements and evaluation criteria

2. IEEE C57.12.24-2016, IEEE Standard for Submersible, Three-Phase Transformers, 3750 kVA

and Smaller: High Voltage, 34 500 GrdY/19 920 Volts and Below; Low Voltage, 600 Volts and

Below

• See §7.5.1.3 for enclosure material thickness requirements

3. IEEE C57.12.40-2017, IEEE Standard for Network, Three-Phase Transformers, 2500 kVA and

Smaller; High Voltage, 34 500 V and Below; Low Voltage, 600 V and Below; Subway and Vault

Types (Liquid Immersed)

• See §5.2.1 for enclosure material thickness requirements

4. ASTM B117-18, Standard Practice for Operating Salt Spray (Fog) Apparatus

5. ASTM B117-19, Standard Practice for Operating Salt Spray (Fog) Apparatus

6. AkzoNobel Coatings Inc Accelerated Weathering / Physical Testing Laboratory Test Reports

• 1032 Hour Test (report dated 10/16/2019), Project 231856 – Interim Report for GE

• 1500 Hour Test (report dated 11/6/2019), Project 231856 – Final Report for GE

• 3000 Hour Test (report dated 5/21/2020), Project 277879 – Final Report for GE

REVISION#1: The original revision of this test report stated that AISI 409L grade stainless-steel

was tested rather than AISI 409 and listed this difference as a deviation versus the

standard. Upon further verification it was identified that AISI 409 was actually

used. For this reason, the lab test reports incorrectly refer to the test panels as

being 409L. This error does not affect the test results or appreciably affect the

conclusions of this report, but this is a substantial revision to the report and must

be noted.

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