comparison of accelerated corrosion tests to corrosion performance in natural atmospheric...

8/8/2019 Comparison of Accelerated Corrosion Tests to Corrosion Performance in Natural Atmospheric Environments

http://slidepdf.com/reader/full/comparison-of-accelerated-corrosion-tests-to-corrosion-performance-in-natural 1/24

COMPARISON OF ACCELERATED CORROSION TESTS TO CORROSION

PERFORMANCE IN NATURAL ATMOSPHERIC ENVIRONMENTS

R. SugamotoUniversity of Hawaii at Manoa

Department of Mechanical [email protected]

G.A. HawthornUniversity of Hawaii at Manoa

Department of Mechanical [email protected]

L.H. HiharaUniversity of Hawaii at Manoa

Department of Mechanical Engineering

[email protected]

ABSTRACT

There is interest in the comparison of accelerated corrosion tests to corrosion performancein natural atmospheric environments. Currently, there are some concerns that acceleratedcorrosion testing may not accurately predict performance in natural atmospheric environments.This provided motivation to compare the corrosion behavior of Al 1060, Al 6061-T6, Al 7075-T6,Al 2024-T3, pure copper, pure magnesium, coated pure magnesium, 1008 steel and pure zincexposed in a variety of natural atmospheric environments such as rainforest, marine, arid,volcanic and light industrial to a modified GM 9540P1 cyclic-corrosion test. The modified GM

9540P cyclic-corrosion test was run for 6, 24, 36 and 48 cycles. Results indicated a lack of agreement in the ordering of alloy performance between the accelerated tests andperformance in the natural atmospheric environments. Hence, the use of acceleratedcorrosion testing may not always result in the optimal selection of materials for field use. Thedata generated also provide the equivalent days of outdoor exposure per cycle of the modifiedGM 9540P test for all of the alloys. Corrosion rates were also determined as a function of thenumber of test cycles, showing for which alloys and metals can corrosion rates be predictedwith confidence.

1



KEY WORDS

Accelerated Corrosion Testing, Atmospheric Corrosion Testing, Outdoor Exposure, CyclicCorrosion Testing, GM9540P, Aluminum, Copper, Magnesium, Steel, Zinc, Rainforest, Marine,Arid, Volcanic, Light Industrial, Comparison, Corrosion Performance, Predict

INTRODUCTION

Accelerated tests such as the General Motors 9540P were developed to measure thecosmetic effects of corrosion of automotive painted, or coated, steel surfaces; however, their success at evaluating the performance of these finishes by relative rank has led to wider use.2 It is important, however, to understand the aggressiveness of the accelerated tests and thecorrelation to results obtained by field testing.

In order to gain a better understanding of this correlation, studies were initiated by theHawaii Corrosion Laboratory to compare corrosion data obtained in a wide array of naturalenvironments to those obtained through accelerated testing. Hawaii’s diverse climate providedthe opportunity to collect data in arid, light industrial, marine and rainforest environments.

The coupons exposed outdoors were installed at Hawaii Corrosion Laboratory test sites onOahu, Hawaii. The test sites are representative of light industrial (Campbell Industrial Park),marine (Coconut Island and Kahuku), arid (Ewa Nui and Waipahu) and rainforest (LyonArboretum) environments.

The salt solutions used for testing consisted of the standard GM9540P solution and a 0.5Msodium sulfate (Na2SO4) solution. The sodium sulfate solution was used to determine theeffects of a chloride-free electrolyte versus the standard chloride-containing GM9540P solution.Previous studies conducted at the Hawaii Corrosion Laboratory have compared Al 6061-T6coupled to various ceramic materials exposed at the outdoor test sites to the same specimen

configurations exposed in a humidity chamber, where specimens were first dipped in sodiumsulfate or other chloride-containing electrolytes.3 The results for the sodium sulfate electrolytehad a better correlation to the outdoor studies than the sodium chloride electrolyte. Thestandard GM9540P solution was used for the 6, 24, 36 and 48-cycle tests and the sodiumsulfate solution was used for a 24-cycle test.

EXPERIMENTAL PROCEDURE

Specimens

The specimens (also referred to as “coupons”) chosen for this study include aluminum,copper, magnesium, steel and zinc substrates of various types, finishes and sizes (Figure 1and Table 1).

2



grade B6

zinc

1060

aluminum1008

steel

CA-110

copper

2024-T3

aluminum

6061-T6

aluminum

7075-T6

aluminum

AZ31-B

magnesium

AZ31-B

magnesium*

* chemical

conversion

coating

grade B6

zinc

1060

aluminum1008

steel

CA-110

copper

2024-T3

aluminum

6061-T6

aluminum

7075-T6

aluminum

AZ31-B

magnesium

AZ31-B

magnesium*

* chemical

conversion

coating

FIGURE 1 - Specimen types

Each coupon was pinstamped, degreased, and weighed prior to being exposed. Threecoupons of each material were included in the exposure sets.

TABLE 1 - Specimen Exposure Schedule

3 month 6 month 12 month 6 cycle 24 cycle 36 cycle 48 cycle Na2SO4

Aluminum 1060 101.6 50.8 2.0 X X X X X X

Aluminum 2024-T3 50.8 25.4 2.0 X X X X X X


Aluminum 6061-T6 50.8 50.8 3.2 X X X


Copper CA-110 101.6 50.8 1.7 X X X X X X

Magnesium AZ-31B 50.8 25.4 2.4 X X X X X X

MagnesiumAZ-31B w/ Dow 7

conversion coating 50.8 25.4 2.4 X X X X X X

Steel 1008 101.6 50.8 1.8 X X X X X X

Zinc Grade B6 101.6 50.8 2.5 X X X X X X

Exposure

Material Type/Finish/Coating

Coupon Size

L (mm) W (mm) T (mm)Outdoor Test Site Modified GM9540P

The set of 50.8 x 50.8 x 3.2 mm Al 6061-T6 coupons were previously exposed at the same

test sites under similar conditions for 3, 6 and 12 months4 and will be used as a basis for comparison.

Mounting Configuration

Non-conductive Delrin® insulators were used to mount coupons onto portable exposureracks (PERs) that were fabricated from Al 6061-T6. A 316 stainless steel splash guard wasinstalled on each PER to prevent cross contamination of copper ions onto other alloy

specimens (Figure 2).

Atmospheric Test Sites. The PERs were mounted onto support structures for exposure of coupons at a 45-degree angle from the horizontal and oriented to face prevailing winds (North-East).

Cyclic Corrosion Test Chamber. Face plates were mounted at a 45-degree angle from thehorizontal within the chamber.

3



(a) (b)

FIGURE 2 – Examples of mounting configurations at (a) outdoor test sites and (b) within thecyclic corrosion test chamber.

Exposure Conditions

Atmospheric Test Sites. The coupons exposed outdoors were installed at Hawaii

Corrosion Laboratory test sites on Oahu, Hawaii. Characteristics of the test sites are provide(Tables 2 and 3).

TABLE 2 - Hawaii Corrosion Laboratory Oahu Test Sites

Campbell Industrial Park Coconut Island

- Light industrial environment - Marine environment

- Low humidity and rainfall - High humidity and rainfall

- Presence of sulfur - Low time of wetness

- Moderate chloride ion concentrations - High chloride ion concentrations

Ewa Nui Kahuku

- Agricultural environment - Marine environment- Low humidity and rainfall - High chloride ion concentrations

- Low time of wetness

- Low chloride ion concentrations

Lyon Arboretum Waipahu

- Rainforest climate - Dry climate

- High humidity and rainfall - Low humidity and rainfall

- High time of wetness - Low time of wetness

- Low chloride ion concentrations - Low chloride ion concentrations

4



TABLE 3 – Average Atmospheric Data for Oahu Test Sites5

Temperature(°C)

RelativeHumidity (%)

ChlorideDeposition Rate

(mg/m2/day)

Time of Wetness (%)

Campbell Industrial Park 27.4 64.9 21.9 13.0

Coconut Island 26.2 76.5 51.3 14.0

Ewa Nui 25.8 68.8 0.0 10.6Kahuku 26.5 78.7 71.0 21.5

Lyon Arboretum 23.1 86.6 0.0 26.3

Waipahu 26.7 67.8 2.6 9.9

Cyclic Corrosion Test Chamber. A Singleton CCT-10 cyclic corrosion test chamber (CCTC) was used to perform the test. Modified versions of the GM9540P cyclic corrosion testwere performed. The modifications refer to the mounting angle being 45-degrees from thehorizontal in all tests and in one test modifications were made to the salt solution.

The salt solutions used for the testing consists of the 1) standard GM9540P solution; 0.9%sodium chloride (NaCl), 0.1% calcium chloride (CaCl2), 0.25% sodium bicarbonate (NaHCO3);and 2) a 0.5M sodium sulfate (Na2SO4) solution.

The standard GM9540P solution was used for the 6, 24, 36 and 48-cycle tests and thesodium sulfate solution was used for a 24-cycle test. The environmental parameters for all of the tests are listed in the table below.

TABLE 4 - List of environmental parameters for the GM9540P test.

Temp RH

1 Salt mist application (15 sec) 00:00 1 25C -

2 Purge chamber 00:01 5 - -3 Humidity control 00:06 84 25C 50%


5 Purge chamber 01:31 5 - -

6 Humidity control 01:36 84 25C 50%







13 Fog 08:00 480 49C -

14 Dry 16:00 460 60C 10%15 Purge chamber 23:40 20 - -

DescriptionStep SetpointsDuration

(min)

Start Time

5



Typical GM9540P Temperature and Humidity Cycle

20

30

40

50

60

70

80

90

100

0:00 2:00 4:00 6:00 8:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0:00

Time

Relative Humidity (%)

Cabinet Temperature (C)

FIGURE 3 - Typical temperature and humidity plots for one cycle of the GM9540P test.

Specimen Cleaning and Processing

Coupons were cleaned in accordance with ISO 8407:1991(E)6 except for the steel couponsexposed to modified GM9540P tests. Corrosion products on steel exposed to the 24, 36 and48 cycle modified GM9540P tests were removed using a mild sand blasting process inaccordance with the GM9540P standard. In most cases, several cleanings were required tocompletely remove the corrosion products. After the cleaning process, each coupon wasrinsed, dried and weighed to determine mass loss.

Penetration Rate

After mass loss was determined, calculations for penetration rate were performed.Penetration rates for the 6-month outdoor exposed coupons and the coupons exposed to 6cycles of the modified GM9540P test were based on an average of three coupons. One cycleof the GM9540P test was equated to one day of exposure for purposes of the calculations.

Penetration rates for the coupons exposed to 24, 36 and 48 cycles of the modifiedGM9540P tests were based on an average of two coupons. Only two out of three couponswere available for cleaning and analysis at the time of publication due to delays in X-raydiffraction (XRD) analysis on the third coupon. Once XRD analysis is conducted, the couponswill be cleaned, weighed and added to the data set for penetration rate determination.Therefore, the results should be considered preliminary.

3D Profilometry

Scan Parameters. A MicroPhotonics MicroMeasure 3D non-contact profilometer was usedto obtain 3-dimensional surface maps of the coupons. An optical lens pen with a spot diameter of 25µm and Z (height resolution) of 0.1µm was used for each measurement. The lens penwas used with a scan acquisition rate of 300Hz and lateral step distance of 12.5µm for eachmeasurement. The total scan area was 20 mm x 20 mm, near the center regardless of couponsize.

6



Data Processing. After each coupon was scanned, several operators were applied to the

raw data in order to process and extract pertinent information. A “leveling” operator was firstapplied to level the data onto a flat plane.

Five 5 mm x 5 mm areas near the four corners and center of the total (20 mm x 20 mm)

scan area were enlarged to indentify pits. The surfaces were examined for pits larger than20 µm in diameter and deeper than 50 µm.

When pits were identified, an area of 0.5 mm x 0.5 mm was enlarged and a “thresholding”operator was applied to remove surface roughness in the area surrounding the pit in order tomeasure pit depth.

RESULTS and DISCUSSION

Although nine materials were exposed at the outdoor test sites and the modified GM9540Ptest, only a limited number of specimens will be discussed. Future studies will incorporate theentire data set.

TABLE 5 - Available data for comparison. 3 month 6 month 12 month 6 cycle 24 cycle 36 cycle 48 cycle Na2SO4

Aluminum 1060 101.6 50.8 2.0 P, 3D P, 3D P P P P

Aluminum 2024-T3 50.8 25.4 2.0 P, 3D P, 3D P P P P


Aluminum 6061-T6 50.8 50.8 3.2 P P P


Copper CA-110 101.6 50.8 1.7 P, 3D P, 3D P P P P

Magnesium AZ-31B 50.8 25.4 2.4 P, 3D P, 3D P P P P

Magnesium AZ-31B w/ Dow 7conversion coating 50.8 25.4 2.4 P, 3D P, 3D P P P P

Steel 1008 101.6 50.8 1.8 P, 3D P, 3D P P P P

Zinc Grade B6 101.6 50.8 2.5 P, 3D P, 3D P P P P

Exposure


Coupon Size

L (mm) W (mm) T (mm)Outdoor Test Site Modified GM9540P

P = Penetration rate data available3D = 3D profilometry data available

Corrosion Rate

6-Month Outdoor Corrosion Rates. The table below is a summary of penetration rates for the materials exposed at the outdoor sites.

7



TABLE 8 – Relative penetration rate comparison for coupons exposed for 6 months at theoutdoor test sites and to 6 cycles of the modified GM9540P test.

Steel 1008 0.0302 Steel 1008 0.0258 Steel 1008 0.0190 Steel 1008 0.0293 Mg AZ-31B 0.0492 Steel 1008 0.0207 Steel 1008 2.6578

Mg AZ-31B 0.0116 Mg AZ-31B 0.0214 Mg AZ-31B 0.0136 Mg AZ-31B 0.0213Mg AZ-31B

conv. coat0.0439 Mg AZ-31B 0.0124 Mg AZ-31B 0.7313

Mg AZ-31B

conv. coat0.0085

Mg AZ-31B

conv. coat0.0157

Mg AZ-31B

conv. coat0.0089

Mg AZ-31B

conv. coat0.0158 Steel 1008 0.0352

Mg AZ-31B

conv. coat0.0096

Zinc

Grade B60.2258

Al 2024-T3 0.0029Copper

CA-1100.0031

Copper

CA-1100.0020

Copper

CA-1100.0027

Zinc

Grade B60.0024

Copper

CA-1100.0015

Mg AZ-31B

conv. coat0.1605

Copper

CA-1100.0025

Zinc

Grade B60.0017

Zinc

Grade B60.0010 Al 7075-T6 0.0014

Copper

CA-1100.0019

Zinc

Grade B60.0011 Al 2024-T3 0.1011

Al 7075-T6 0.0019 Al 2024-T3 0.0010 Al 2024-T3 0.0009 Al 2024-T3 0.0010 Al 7075-T6 0.0016 Al 2024-T3 0.0009 Al 7075-T6 0.0764

Zinc

Grade B60.0014 Al 7075-T6 0.0010 Al 7075-T6 0.0007

Zinc

Grade B60.0010 Al 2024-T3 0.0008 Al 7075-T6 0.0007 Al 1060 0.0648

Al 6061-T6 0.0008 Al 6061-T6 0.0003 Al 6061-T6 0.0003 Al 6061-T6 0.0004 Al 6061-T6 0.0003 Al 6061-T6 0.0003 Al 6061-T6 0.0575

Al 1060 0.0007 Al 1060 0.0003 Al 1060 0.0002 Al 1060 0.0003 Al 1060 0.0002 Al 1060 0.0003 Copper CA-110

0.0362

Campbell

Industrial Park

Relative Penetration Rates for Outdoor Test Sites and the 6-cycle Modified GM9540P (mm/yr)

Lyon Arboretum

(Rainforest)

Kahuku

(Marine)

Ewa Nui

(Agricultural)

Coconut Island

(Marine)

6-cycle Modified

GM9540P

Waipahu

(Dry)

HighestRate

Lowest

Rate The steel alloys appeared to behave similarly in the 6-cycle modified GM9540P when

compared to the other materials as did the aluminum. The copper alloys, however, showedsignificantly lower penetration rates relative to the other alloys in the modified GM9540P test.

Comparison Between Accelerated Coupons. The table below presents the materialsexposed to various cycles and solutions of GM9540P, organized by their relative corrosionrates.

TABLE 9 – Relative penetration rate comparison for coupons exposed to the modified GM9540P tests.

Steel 1008 2.6578 Steel 1008 2.8285 Steel 1008 2.7538 Steel 1008 1.9744 Steel 1008 0.6647

Mg AZ-31B 0.7313 Mg AZ-31B 1.1358 Mg AZ-31B 0.5003 Mg AZ-31B 0.4997Zinc

Grade B60.4629

Zinc

Grade B60.2258

Zinc

Grade B60.2707

Mg AZ-31B

conv. coat0.1779

Zinc

Grade B60.1291 Mg AZ-31B 0.2433

Mg AZ-31B

conv. coat0.1605 Al 2024-T3 0.1565

Zinc

Grade B60.1361

Mg AZ-31B

conv. coat0.1197

Mg AZ-31B

conv. coat0.0718

Al 2024-T3 0.1011

Mg AZ-31B

conv. coat 0.1152 Al 2024-T3 0.0787 Al 2024-T3 0.0976

Copper

CA-110 0.0084

Al 7075-T6 0.0764Copper

CA-1100.0340

Copper

CA-1100.0268

Copper

CA-1100.0228 Al 7075-T6 0.0062

Al 1060 0.0648 Al 7075-T6 0.0276 Al 7075-T6 0.0181 Al 7075-T6 0.0154 Al 6061-T6 0.0055

Al 6061-T6 0.0575 Al 1060 0.0177 Al 1060 0.0134 Al 1060 0.0088 Al 2024-T3 0.0053

Copper

CA-1100.0362 Al 6061-T6 0.0176 Al 6061-T6 0.0101 Al 6061-T6 0.0068 Al 1060 0.0052

Highest

Rate

Lowest

Rate

Relative Penetration Rates for the Modified GM9540P tests (mm/yr)

6-cycle 24-cycle 36-cycle 48-cycleNa2SO4

24-cycle

9



The modified GM9540P test using sodium sulfate as the electrolyte produced results whichwere very different from the other GM9540P tests using the standard solution. The penetrationrates overall were significantly lower for the 24-cycle sodium sulfate test when compared to the24-cycle test using the standard GM9540P solution and the relative rankings of copper, zincand Al 2024-T3 were also different.

Comparative Ratios for the Accelerated Tests. Using the steel penetration rates as a basisfor comparison, ratios were calculated to determine which materials performed consistentlythroughout the test sites. Table 10 presents the penetration rate for steel at each site dividedby the penetration rates for the other materials at the same site. Table 11 presents thepenetration rate for steel during each GM9540P test divided by the penetration rates for theother materials exposed to the same test. Steel was chosen as the basis for this comparisonsince the GM9540P test was developed to evaluate painted or coated steel surfaces.

The penetration rate ratios for each material exposed to the outdoor test sites in Table 10were divided by the corresponding ratio for the GM9540P tests in Table 11 to identify whichsites and materials were in line with the accelerated test. The results are presented in Tables12 through 16 below. Ratios that are close to 1.0 indicate materials and test sites that theGM9540P test represents closely based upon comparison to the penetration rates for steel.

TABLE 10 – Ratio of penetration rates for steel to other materials for couponsexposed for 6 months at the outdoor test sites.

Aluminum 1060 44.3 98.3 81.5 101.9 141.3 82.4

Aluminum 2024-T3 10.4 26.2 20.2 28.0 45.6 23.7

Aluminum 6061-T6 39.4 97.8 61.7 83.2 124.9 61.9

Aluminum 7075-T6 16.0 26.7 25.8 20.5 22.6 29.5

Copper CA-110 12.2 8.2 9.4 11.0 18.3 14.2

Magnesium AZ-31B 2.6 1.2 1.4 1.4 0.7 1.7


conversion coating3.6 1.6 2.1 1.9 0.8 2.2

Steel 1008 1.0 1.0 1.0 1.0 1.0 1.0

Zinc Grade B6 21.9 15.2 19.6 30.5 14.6 19.7

RATIO OF STEEL PENETRATION RATE TO OTHER MATERIALS

Material Type/Finish/Coating Campbell

Industrial Park

Coconut Island

(Marine)

Ewa Nui

(Agricultural)

Kahuku

(Marine)

Lyon Arboretum

(Rainforest)Waipahu (Dry)

10



TABLE 11 – Ratio of penetration rates for steel to other materialsfor coupons exposed to the modified GM9540P test.

Aluminum 1060 41.0 159.5 206.0 224.7 127.1

Aluminum 2024-T3 26.3 18.1 35.0 20.2 126.4

Aluminum 6061-T6 46.3 160.5 273.1 289.8 120.5

Aluminum 7075-T6 34.8 102.6 152.5 127.9 106.7

Copper CA-110 73.4 83.2 102.8 86.5 79.0

Magnesium AZ-31B 3.6 2.5 5.5 4.0 2.7


conversion coating16.6 24.6 15.5 16.5 9.3

Steel 1008 1.0 1.0 1.0 1.0 1.0

Zinc Grade B6 11.8 10.4 20.2 15.3 1.4

36-cycle Modified

GM9540P

48-cycle Modified

GM9540P

6-cycle Modified

GM9540P

24-cycle Modified

GM9540P

24-cycle Na2SO4

Mod. GM9540P

RATIO OF STEEL PENETRATION RATE TO OTHER MATERIALS


TABLE 12 – Ratio between outdoor sites and 6-cycle GM9540P

with steel corrosion rates as an overall basis for relative comparison

Aluminum 1060 1.08 2.40 1.99 2.48 3.45 2.01 2.23 0.77

Aluminum 2024-T3 0.40 1.00 0.77 1.07 1.74 0.90 0.98 0.44

Aluminum 6061-T6 0.85 2.11 1.33 1.80 2.70 1.34 1.69 0.66

Aluminum 7075-T6 0.46 0.77 0.74 0.59 0.65 0.85 0.68 0.14

Copper CA-110 0.17 0.11 0.13 0.15 0.25 0.19 0.17 0.05

Magnesium AZ-31B 0.72 0.33 0.39 0.38 0.20 0.46 0.41 0.17

Magnesium AZ-31B w/ Dow 7conversion coating

0.22 0.10 0.13 0.11 0.05 0.13 0.12 0.05

Zinc Grade B6 1.86 1.29 1.66 2.59 1.24 1.67 1.72 0.49

Average 0.72 1.01 0.89 1.15 1.28 0.94

Standard Deviation 0.56 0.88 0.70 1.02 1.26 0.69

Campbell

Industrial

Park

Type/Finish/CoatingMaterialWaipahu

(Dry)

Lyon

Arboretum

(Rainforest)

Kahuku

(Marine)

Std.

Dev.Avg.

Ewa Nui

(Agricultural)

Coconut

Island

(Marine)

11



TABLE 13 – Ratio between outdoor sites and 24-cycle GM9540Pwith steel corrosion rates as an overall basis for relative comparison

Aluminum 1060 0.28 0.62 0.51 0.64 0.89 0.52 0.57 0.20

Aluminum 2024-T3 0.57 1.45 1.12 1.55 2.52 1.31 1.42 0.64

Aluminum 6061-T6 0.25 0.61 0.38 0.52 0.78 0.39 0.49 0.19

Aluminum 7075-T6 0.16 0.26 0.25 0.20 0.22 0.29 0.23 0.05

Copper CA-110 0.15 0.10 0.11 0.13 0.22 0.17 0.15 0.04

Magnesium AZ-31B 1.05 0.49 0.56 0.55 0.29 0.67 0.60 0.25


0.15 0.07 0.09 0.08 0.03 0.09 0.08 0.04

Zinc Grade B6 2.10 1.45 1.88 2.91 1.40 1.89 1.94 0.55

Average 0.59 0.63 0.61 0.82 0.79 0.66


Coconut

Island

(Marine)

Campbell

Industrial

Park

RATIO BETWEEN PERFORMANCE IN THE 24-CYCLE GM 9540P AND OUTDOOR SITES

Waipahu

(Dry)

Lyon

Arboretum

(Rainforest)

Kahuku

(Marine)

Ewa Nui

(Agricultural)Avg.

Std.

Dev.Material Type/Finish/Coating

TABLE 14 – Ratio between outdoor sites and 36-cycle GM9540P


Aluminum 1060 0.22 0.48 0.40 0.49 0.69 0.40 0.44 0.15

Aluminum 2024-T3 0.30 0.75 0.58 0.80 1.30 0.68 0.73 0.33

Aluminum 6061-T6 0.14 0.36 0.23 0.30 0.46 0.23 0.29 0.11

Aluminum 7075-T6 0.10 0.17 0.17 0.13 0.15 0.19 0.15 0.03

Copper CA-110 0.12 0.08 0.09 0.11 0.18 0.14 0.12 0.04

Magnesium AZ-31B 0.47 0.22 0.25 0.25 0.13 0.30 0.27 0.11


conversion coating0.23 0.11 0.14 0.12 0.05 0.14 0.13 0.06

Zinc Grade B6 1.08 0.75 0.97 1.50 0.72 0.97 1.00 0.28

Average 0.33 0.36 0.35 0.46 0.46 0.38



Waipahu

(Dry)

LyonArboretum

(Rainforest)

Kahuku

(Marine)

Ewa Nui

(Agricultural)

CoconutIsland

(Marine)

CampbellIndustrial

Park

Material Avg.Std.

Dev.Type/Finish/Coating

12



TABLE 15 – Ratio between outdoor sites and 48-cycle GM9540Pwith steel corrosion rates as an overall basis for relative comparison

Aluminum 1060 0.20 0.44 0.36 0.45 0.63 0.37 0.41 0.14

Aluminum 2024-T3 0.51 1.30 1.00 1.39 2.26 1.17 1.27 0.57

Aluminum 6061-T6 0.14 0.34 0.21 0.29 0.43 0.21 0.27 0.11

Aluminum 7075-T6 0.13 0.21 0.20 0.16 0.18 0.23 0.18 0.04

Copper CA-110 0.14 0.09 0.11 0.13 0.21 0.16 0.14 0.04

Magnesium AZ-31B 0.66 0.31 0.35 0.35 0.18 0.42 0.38 0.16


0.22 0.10 0.13 0.11 0.05 0.13 0.12 0.05

Zinc Grade B6 1.43 0.99 1.28 1.99 0.96 1.29 1.32 0.38

Average 0.43 0.47 0.46 0.61 0.61 0.50



Waipahu

(Dry)

Lyon

Arboretum

(Rainforest)

Kahuku

(Marine)

Ewa Nui

(Agricultural)

Coconut

Island

(Marine)

Campbell

Industrial

Park

Avg.Std.

Dev.Material Type/Finish/Coating

TABLE 16 – Ratio between outdoor sites and 24-cycle sodium sulfate (Na2SO4) GM9540P


Aluminum 1060 0.35 0.77 0.64 0.80 1.11 0.65 0.72 0.25

Aluminum 2024-T3 0.08 0.21 0.16 0.22 0.36 0.19 0.20 0.09

Aluminum 6061-T6 0.33 0.81 0.51 0.69 1.04 0.51 0.65 0.25

Aluminum 7075-T6 0.15 0.25 0.24 0.19 0.21 0.28 0.22 0.05

Copper CA-110 0.15 0.10 0.12 0.14 0.23 0.18 0.15 0.05

Magnesium AZ-31B 0.95 0.44 0.51 0.50 0.26 0.61 0.55 0.23


conversion coating0.39 0.18 0.23 0.20 0.09 0.23 0.22 0.10

Zinc Grade B6 15.28 10.57 13.65 21.21 10.17 13.72 14.10 4.00

Average 2.21 1.67 2.01 2.99 1.68 2.05


Waipahu

(Dry)Avg.

Std.

Dev.

RATIO BETWEEN PERFORMANCE IN THE 24-CYCLE Na2SO4 GM9540P AND OUTDOOR SITES


Campbell

Industrial

Park

Coconut

Island

(Marine)

Ewa Nui

(Agricultural)

Kahuku

(Marine)

Lyon

Arboretum

(Rainforest)

13



With respect to steel, the ratios in Tables 11 through 14 suggest correlation with Al 2024-T3at most of the sites and reasonable correlation for Al 6061-T6 and Al 7075-T6 at some of thesites. The ratios also infer weak or no correlations between the GM9540P test for Al 1060,copper, and magnesium with the Dow 7 chemical conversion coating. The ratios also indicatevery little correlation between the rainforest environment and the GM9540P test. Correlationoverall had a tendency to drop off as the number of GM9540P cycles increased, however it is

important to note that the correlations are also based upon 6 months of exposure at theoutdoor sites.

Penetration Rate Decay Over Time. The penetration rates were plotted 1) as a function of exposure time at six different outdoor test sites for Al 6061-T6 (Figure 4), and 2) as a functionof cycles exposed in the modified GM9540P test for all of the alloys (Figures 5 and 6). Resultswere plotted on a log scale.

The slope (m) of the curves varied from approximately more than -0.1 to less than -1.Ideally, a slope of zero would indicate linear corrosion-rate kinetics; and a slope of -0.5 wouldindicate parabolic corrosion-rate kinetics, where the corrosion rate decreases with time as aprotective corrosion film thickens, slowing the diffusion of ions or migration of electrons. For the set of curves corresponding to the outdoor exposure of Al 6061-T6 (Figure 4), thecorrosion rate would also be dependent on weather parameters which may not be consistentover the exposure period; and hence, the slope of the curve would be influenced by other factors. For Al 6061-T6 at the outdoor sites, the slopes varied from approximately -0.6 to -1.Aside from weather parameters, a hypothesis for slopes less than -0.5 is that corrosion ratesfurther decrease as relatively large cathodic precipitates are undercut and dislodged from themicrostructure, rendering the alloys less prone to corrosion by elimination of cathodic sites.The cases in which the slopes were close to -1 were at Coconut Island and Kahuku wherechloride deposition was relatively high. Interestingly, slopes close to -1 were also observed for Al 6061-T6 and Al 1060 exposed in the modified GM9540P test where their corrosion rates

significantly decreased with exposure cycles in comparison to Al 7075-T6 and Al 2024-T3(Figure 5). The slope for Al 2024-T3 was -0.05, indicating a tendency towards linear corrosion-rate kinetics, which could be due to its relatively high copper content (i.e., 4.4%). The other alloys also showed more tendency towards linear corrosion-rate kinetics: Mg (m = -0.17),coated Mg (m = -0.08), Cu (m = -0.20), Zn (m = -0.26), and 1008 steel (m = -08). It is alsoimportant to note that the R2 values for the aluminum alloys were generally greater than 0.9except for the case of Al 2024-T6 which was very low. The R2 values for the other alloysshowing a tendency towards linear corrosion-rate kinetics were also low, making accurateprediction of corrosion rates by interpolation or extrapolation difficult.

14



Al 6061-T6 Penetration Rates at

Campbell Industrial Park

y = -0.626x - 2.8217

R2 = 0.9997

-4.50

-3.75

-3.00

-2.25

-1.50

-0.75

0.00

0.0 0.2 0.4 0.6 0.8 1.0 1.2

log t [months]

l o g C R [ m m / y

]


Coconut Island

y = -0.9719x - 2.7061

R2 = 0.9869

-4.50

-3.75

-3.00

-2.25

-1.50

-0.75

0.00

0.0 0.2 0.4 0.6 0.8 1.0 1.2

log t [months]

l o g C R [ m m / y

]


Ewa Nui

y = -0.6635x - 3.1396

R2 = 0.965

-4.50

-3.75

-3.00

-2.25

-1.50

-0.75

0.00

0.0 0.2 0.4 0.6 0.8 1.0 1.2

log t [months]

l o g C R [ m m / y ]


Kahuku

y = -1.1852x - 2.6458

R2 = 0.9499

-4.50

-3.75

-3.00

-2.25

-1.50

-0.75

0.00

0.0 0.2 0.4 0.6 0.8 1.0 1.2

log t [months]

l o g C R [ m m / y ]


Lyon Arboretum

y = -0.7055x - 3.0538

R2 = 0.7756

-4.50

-3.75

-3.00

-2.25

-1.50

-0.75

0.00

0.0 0.2 0.4 0.6 0.8 1.0 1.2

log t [months]

l o

g C R [ m m / y ]


Waipahu

y = -0.8403x - 2.8234

R2 = 0.9995

-4.50

-3.75

-3.00

-2.25

-1.50

-0.75

0.00

0.4 0.6 0.8 1.0 1.2

log t [months]

l o

g C R [ m m / y ]

FIGURE 4 – Penetration rates for Al 6061-T6 exposed for 3, 6 and 12 months at the outdoor test sites (plotted on log scales).

15



Al 1060 Penetration Rates for

Modified GM9540P

y = -0.933x - 0.4588

R2 = 0.9947

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.0 0.5 1.0 1.5 2.0

log N [# of cycles]

l o g C R [ m m / y ]

Al 2024-T3 Penetration Rates for

Modified GM9540P

y = -0.045x - 0.9181

R2 = 0.0207

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.0 0.5 1.0 1.5 2.0

log N [# of cycles]

l o g C R [ m m / y ]


Modified GM9540P

y = -1.0004x - 0.4398

R2 = 0.9857

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.0 0.5 1.0 1.5 2.0

log N [# of cycles]

l o g C R [ m m / y ]


Modified GM9540P

y = -0.7798x - 0.5059

R2 = 0.9961

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.0 0.5 1.0 1.5 2.0

log N [# of cycles]

l o g C R [ m m / y ]

Uncoated Mg Penetration Rates

for Modified GM9540P

y = -0.1732x + 0.063

R2 = 0.167

-1.5

-1.0

-0.5

0.0

0.5

1.0

0.0 0.5 1.0 1.5 2.0

log N [# of cycles]

l o

g C R [ m m / y ]

Coated Mg Penetration Rates for

Modified GM9540P

y = -0.0783x - 0.7456

R2 = 0.1127

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.0 0.5 1.0 1.5 2.0

log N [# of cycles]

l o

g C R [ m m / y ]

FIGURE 5 – Penetration rates for aluminum and magnesium exposed to 6, 24, 36 and 48cycles of the modified GM9540P test (plotted on log scales).

16



Copper Penetration Rates for

Modified GM9540P

y = -0.1969x - 1.2654

R2 = 0.7184

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.0 0.5 1.0 1.5 2.0

log N [# of cycles]

l o g C R [ m m / y

]

1008 Steel Penetration Rates

for Modified GM9540P

y = -0.0809x + 0.512

R2 = 0.1995

0.0

0.1

0.2

0.3

0.4

0.5

0.0 0.5 1.0 1.5 2.0

log N [# of cycles]

l o g C R [ m m

/ y ]

Zinc Penetration Rates for

Modified GM9540P

y = -0.2576x - 0.3947

R2 = 0.4159

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.0 0.5 1.0 1.5 2.0

log N [# of cycles]

l o g C R [ m m / y ]

FIGURE 6 – Penetration rates for copper, steel and zinc exposed to 6, 24, 36 and 48 cycles of the modified GM9540P test (plotted on log scales).

Pit Depth and Pit Density

Aluminum coupons exposed to 6 cycles of the GM9540P test and 6 months at CampbellIndustrial Park (CIP) and Coconut Island (CI) were scanned using a 3D profilometer. Virginspecimens were also scanned as a baseline for comparison.

In most cases, two types of pits were found; pits that were circular in shape and pits thatappeared as trenches. The “trench” pits most likely grew from within scratches on the couponsurface, therefore it is difficult to estimate the true depth of the trench pits. Pits were alsofound on virgin specimens, which were likely to have formed during the cleaning process.

Pits were only considered significant if they were deeper than 50 µm and larger than 20 µmin diameter. “Trench” type pits were not used to compare GM9540P to the outdoor sites sincetheir growth due to corrosion was difficult to estimate.

17



TABLE 17 – Pit Summary For Aluminum

Depth

(µm)

Diameter

(µm)

Depth

(µm)

Diameter

(µm)

GM9540P 107 20 107 20 107 20 0.016

CIP - - - - - - -No pits deeper than 50µm

other than trenches

CI - - - - - - -No pits deeper than 50µm

other than trenches

virgin 115 50 115 50 84 38 0.016

GM9540P 67 220 109 25 69 56 0.128

CIP 50 220 87 25 63 39 0.128

CI 130 20 130 20 130 20 0.008

virgin 86 25 86 25 86 25 0.016

GM9540P - - - - - - -No pits deeper than 50µm

other than trenches

CIP 132 50 132 50 110 30 0.024

CI 142 20 142 20 142 20 0.008

virgin 107 30 107 30 107 30 0.008Pits formed on existing

surface defects

GM9540P - - - - - - -No pits deeper than 50µm

other than trenches

CIP - - - - - - -No pits deeper than 50µm

other than trenchesCI 162 40 162 40 162 40 0.008

virgin 52 20 52 20 52 20 0.016

* Only pits deeper than 50µm considered

** Pits deeper than 50µm, Total scan area = 125mm2

Average

Depth

(µm)

Average

Diameter

(µm)

1 0 6 0

2 0 2 4 - T 3

6 0 6 1 - T 6

7 0 7 5

- T 6

Exposure

NotesPit

Density

(mm-2

)

Largest Pit* Deepest Pit*

18



1060 aluminum. Only a few pits were found on the 1060 aluminum coupons and themaximum pit depth found on the corroded coupons were similar in depth to pits found on thevirgin specimen.

(6-cycle GM9540P) (6-month Campbell Industrial Park)

(6-month Coconut Island) (Virgin Specimen)

FIGURE 7 – Examples of pits found on 1060 aluminum.

Al 2024-T3. A significant amount of pits were found on the 2024-T3 coupons compared tothe other alloys. Pits with large diameters and shallow depth were found on the coupons

exposed to GM9540P and at Campbell Industrial Park. Most of the deeper pits were small indiameter.

19



(6-cycle GM9540P) (6-cycle GM9540P)

(6-month Campbell Industrial Park) (6-month Campbell Industrial Park)


FIGURE 8 – Examples of pits found on Al 2024-T3.

20



Al 6061-T6. All of the pits found on the GM9540P coupons were trench type pits and werenot used for comparison. There were few pits found on the other coupons. The pits that werefound were considerably deep, however, similar pits were also found on the virgin specimen.




21



Al 7075-T6. All of the pits found on the GM9540P coupons were trench type pits and werenot used for comparison. A very deep pit was found on the coupon exposed at the CoconutIsland site, roughly 3 times deeper than those found on the virgin specimen.




CONCLUSIONS

Penetration Rate

The penetration rates for specimens exposed at the outdoor test sites generally followed atrend, however, when the relative rates were compared to the modified GM9540P tests,several materials did not follow the same trend.

When considering the relative penetration rates in Table 8 (6-month outdoor and 6-cycleGM9540P tests), copper and zinc in particular did not seem to follow the same trend relative tothe other materials. With Lyon Arboretum as the exception, the penetration rate for copper

22



ACKNOWLEDGMENTS

The authors would like to acknowledge the following for their contributions:

• Mr. Robert Zanowicz, US Army ARDEC, Contract Officer’s Representative and ProgramManager for the Pacific Rim Corrosion Research Program and Pacific Rim

Environmental Degradation of Materials Research Program

• Mr. Lance Miyahara, Hawaiian Electric Company.

• Mr. Alex Niemi, UH Manoa (MS 2005).

REFERENCES

1. General Motors Engineering Standards, Accelerated Corrosion Test, GM9540P, Dec,1997.

2. Raymund Singleton, “Cabinet Testing,” in ASM Handbook Volume 13A Corrosion:Fundamentals, Testing, and Protection, ASM International, 2003, p. 470

3. R. Srinivasan, "Corrosion Studies Between Ceramics and 6061-T6 Aluminum Interface,"M.S. Thesis, University of Hawaii at Manoa, 2005.

4. G.A. Hawthorn, M. Nullet, R. Srinivasan, L.H. Hihara, “Corrosion Testing andAtmospheric Monitoring in an Active Volcanic Environment,” Tri Services Corrosion

Conference, Denver, Colorado, 2007.5. G.A. Hawthorn, L.H. Hihara, “Corrosivity Mapping of the Pacific Theater of Operations,”

NACE 2008 , New Orleans, Louisiana, 2008.6. ISO 8407:1991(E) Corrosion of Metals and Alloys – Removal of Corrosion Products

from Corrosion Test Specimens

comparison of accelerated corrosion tests to corrosion performance in natural atmospheric...

Documents