Accelerated Corrosion Testing (ACT) of external pipeline coatings traditionally has been associated with the ASTM(1) B117 Salt Fog Test1 (which is also similar to other standards like the BS3900 2 Parts F4 and F12 and the ISO standard 72533). There are other types of ACT for coatings that use "salt spray" type tests; for example, the acidified salt spray (ASS) and copper acidified salt spray (CASS) tests, which use test solutions containing acetic acid and copper chloride with acetic acid. Recently, ACT of coatings has been performed using corrosive gases in a closed environment at constant temperature and with varying relative humidity (RH). This test resembles the standard ACT for Electronic Equipment and Devices.4 The objective of these (sometimes cyclic) tests is to try to mimic naturally occurring wetting and drying cycles with the addition of a known acceleration factor.4 The standard ACT tests are important in the development of new coatings and are commonly used to qualify new coatings and surface treatments for novel material/environmental applications. Common practices employ rapid "round robin ACT testing" to down select coating systems from a large number of candidate systems before proceeding with additional testing (including laboratory testing and/or field testing). Unfortunately, the standard ACT tests have been shown to inaccurately predict the performance of coatings in real service conditions.5 Moreover, utilizing ACT during preliminary robin testing could lead to inaccurate rankings. This paper aims to comment on common ACT methods and their accuracy, and to identify those ACT methods that, with additional development, could more realistically mimic service environments, and therefore more accurately predict long-term performance. Additionally, this work describes testing on coatings used for both internal and external coating of pipelines. Of particular interest, the specific ranking between mechanical testing and corrosion testing will be discussed.

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2010

CHARACTERIZATION OF INTERNAL AND EXTERNAL COATINGS USED ON STEEL PIPELINES

S. A. Waters, H. Tsaprailis, W. Kovacs III, J. R. Tuggle, and L. F. Garfias-Mesias

DNV Columbus, Inc.

Corrosion and Materials Technology Laboratory

5777 Frantz Road

Dublin, OH 43017-1386 USA

Phone (614) 761-1214 Fax (614) 761-1633

Steven.Waters@dnv.com

ABSTRACT

Accelerated Corrosion Testing (ACT) of external pipeline coatings traditionally has been associated

with the ASTM

(1)

B117 Salt Fog Test

1

(which is also similar to other standards like the BS3900

2

Parts

F4 and F12 and the ISO standard 7253

3

). There are other types of ACT for coatings that use "salt

spray" type tests; for example, the acidified salt spray (ASS) and copper acidified salt spray (CASS)

tests, which use test solutions containing acetic acid and copper chloride with acetic acid. Recently,

ACT of coatings has been performed using corrosive gases in a closed environment at constant

temperature and with varying relative humidity (RH). This test resembles the standard ACT for

Electronic Equipment and Devices.

4

The objective of these (sometimes cyclic) tests is to try to mimic

naturally occurring wetting and drying cycles with the addition of a known acceleration factor.

4

The standard ACT tests are important in the development of new coatings and are commonly used to

qualify new coatings and surface treatments for novel material/environmental applications. Common

practices employ rapid "round robin ACT testing" to down select coating systems from a large number

of candidate systems before proceeding with additional testing (including laboratory testing and/or field

testing). Unfortunately, the standard ACT tests have been shown to inaccurately predict the

performance of coatings in real service conditions.

5

Moreover, utilizing ACT during preliminary robin

testing could lead to inaccurate rankings.

This paper aims to comment on common ACT methods and their accuracy, and to identify those ACT

methods that, with additional development, could more realistically mimic service environments, and

therefore more accurately predict long-term performance. Additionally, this work describes testing on

coatings used for both internal and external coating of pipelines. Of particular interest, the specific

ranking between mechanical testing and corrosion testing will be discussed.

Keywords: Coatings, Coatings Testing, Accelerated Testing, Accelerated Corrosion Testing, Corrosion

Testing, Internal Pipeline Coatings, External Pipeline Coatings, Pipeline(s), EIS.

1 ASTM International (ASTM), 100 Barr Harbor Dr., West Conshohocken, PA 19428.

©2010 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE

International, Publications Division, 1440 South Creek Drive, Houston, Texas 77084. The material presented and the views expressed in this paper are

solely those of the author(s) and are not necessarily endorsed by the Association.

1

Paper No.

10038

INTRODUCTION

Coatings have been used to protect pipelines since the turn of the century. There are a number of

coatings that have been accepted by industry standards to be used when trying to protect both the

internal and the external surfaces of pipelines.

External coatings provide a dielectric barrier between the metal pipeline and the corrosive environment

surrounding it (i.e. soil, water, and exposure to the atmospheric environment, including sun radiation).

Coating systems can also provide improved mechanical protection and can act as a sacrificial layer (in

the case of metallic coatings). They are typically selected not only because of the extra protection to

the pipeline, but also based on ease of application, availability, and cost. Several external pipeline

coatings have been employed over the years, including coal tar emulsions, tape wrap systems,

polymeric materials (for example polyethylene based coatings) and fusion bond epoxy (FBE) systems

(which are accepted as the standard coating for most of the pipelines in operation today). Recent work

has involved pre-treatment of surfaces using nanoparticles, intended to further improve coating

performance by modifying the contact interface between the coating and the iron substrate (i.e.,

increase adhesion, adding a cathodic or anodic inhibitor or even adding some pigments that can

release a chemical substance in the event of a scratch).

Internal coatings have been used to improve flow characteristics and to provide a barrier layer between

the flow medium and the pipeline, protecting both the pipeline and the product. A number of different

kinds of coatings have been employed over the years, including FBE and polyethylene. Recent work

has focused on other systems that are more "system-specific," in other words, coatings that target

specific fluids being transported in the pipeline. At the same time, long-term reliability of the coating

and ease of application has been the subject of some recent studies in the area of internal coatings.

Generally, coatings are considered to be 'good' if they offer acceptable adhesion to the substrate, resist

water diffusion and/or penetration, resist cathodic disbondment of the film, have good resistance

against deformation and/or cracking, and resist chemical degradation in the intended environment.

During the coating development process, industry commonly requires ACT such as salt spray tests,

immersion in hot liquids or water, humidity tests, and more recently combination of electrochemical

tests with an aggressive environment to evaluate the properties of the coatings. ACT has also been

used for materials selection and quality control (for example when selecting the material for

construction of a storage tank). As such, tests evaluating these properties have evolved from common

practices to become international standards. However, performance in the field does not always

correlate well with the results obtained in the lab using ACTs based on these international standards

(the results observed from actual field exposures can be significantly different). For example, the

ASTM B117 Salt Spray Test is often used for general assessment of materials and coatings. Variants

of the ASTM B117 test can be used to evaluate coatings and anodized films on aluminum substrates.

These variations include the acidified salt spray (ASS), which utilizes a salt spray with acetic acid, and

the copper acidified salt spray (CASS) that contains copper chloride in the solution. Further test

modifications can also include wet/dry cycling and/or injection of sulfur dioxide or other gases that are

intended to improve accuracy of test results.

While salt spray tests and their intended uses are well documented, the actual usefulness of these

tests has been actively debated. Literature indicates that these tests are not reliable and fail to

reproduce the performance obtained under natural exposure or actual service conditions

5

.

Consequently, there is an overall lack of confidence in accelerated corrosion testing that has resulted in

at least two major drawbacks to the utilization of ACT approaches: (1) Due to the inaccuracies of ACT,

only field trials are acceptable by some end-users, which requires more evaluation time and increased

testing costs and (2) if ACT is used to down select from a large number of candidate materials before

performing long term testing (due to economic considerations or otherwise), inaccurate selection may

occur that would lead to the selection of the wrong candidate materials for long term testing (in other

words, down selection may lead to the elimination of viable coatings for the particular application, while

promoting coating systems which are not desirable).

2

Recently, approaches utilizing a modified ASTM G44 alternate-immersion test have shown promise.

This test method was based on data collected using electrochemical probes, which suggested that

reduced chloride level in the ASTM G44 test caused corrosion mechanisms to correspond better with

those observed during field exposures (marine conditions). Resulting corrosion rankings of metallic

coating systems on steel showed significantly improved correlation with field exposure when compared

with results obtained from standard salt spray testing.

5

In general, when selecting a number of candidate systems for a particular application and round robin

testing, one should take into consideration all the desired properties that the coating should have, and

consider different tests that measure those particular properties.

EXPERIMENTAL

Corrosion Testing

Corrosion testing is by far the most popular (and sometimes the only type of testing) that is considered

when selecting a coating. Two types of corrosion testing are very common, those that measure the

performance of the coating while exposed to an aggressive environment under extreme conditions (like

the salt fog test) and those that measure the resistance to cathodic disbondment (for example, when a

defect is present in the coating, which may lead to the disbondment of the coating in the area adjacent

to the defect). Other types of tests (less commonly used) include the measurement of more specific

properties that will have an impact in the overall performance of the coating or that will indirectly

measure coating defects. Table 1 shows the most common types of tests that are typically referred to

as "corrosion testing."

TABLE 1 Corrosion Tests and Applicable Standards.

Test Applicable Standard

Dielectric Strength ASTM D149

6

Porosity, Interface and Cross-Sectional CSA-Z245.20-06

7

Resistance to Cathodic Disbondment ASTM G8

8

Resistance to Cathodic Disbondment ASTM G429

Resistance to Cathodic Disbondment ASTM G95

10

Resistance to Cathodic Disbondment CSA-Z245.20-06

Resistance to Chemical Degradation NACE TM0174

11

Salt Fog ASTM B117

UV-Condensation ASTM D4587

12

Water Absorption ASTM D870

13

Water Vapor Transmission ASTM D1653

14

Traditional corrosion testing of coatings involves the use of "panels" or "coupons". These coupons are

commercially available and easy to obtain. All panels/coupons used in corrosion tests are required to

be cleaned with blast media to achieve NACE "white metal" blast, and achieve the desired surface

profile for coating (typically ~2-3 mil anchor pattern).

15

In all cases, coatings should be applied in

accordance with the manufacturer's instructions using certified coating applicators to ensure accurate

testing. In the event that a subcontractor is used to apply the coating, certification or third party

witnessing should be required. This ensures that the coating is applied in accordance with the

guidelines and regulations stated in most of the international recommended practices and/or

international standards. This is not typically needed when the original manufacturer of the coating

applies the coating to the panels/coupons prior to testing.

3

Mechanical Testing

Mechanical tests are typically the "secondary" source of information when it comes to coating system

selection. However, it is important that most of these properties be assessed prior to selecting a

coating, as they offer fundamental insight relating to coating system performance. One of the most

important properties, adhesion, plays a critical role in the long term durability (reliability) of a coating.

Despite the relevance of the tests, however, they are frequently overlooked or, even if considered,

dismissed due to budget restriction or for other reasons. Table 2 shows some of the Mechanical tests

and standards that are typically performed when selecting a coating.

TABLE 2 Mechanical Tests and Applicable Standards.

Test Applicable Standard

Abrasion Resistance ASTM D4060

16

Adhesion to Steel (Pull-Off Adhesion) ASTM D4541

17

Durometer Hardness (Shore D) ASTM D2240

18

Elongation +Tensile Strength ASTM D638

19

Flexibility of Coating/Bendability CSA-Z245.20-06

20

Lap Shear Adhesion ASTM D1002

21

Resistance to Cracking ASTM D522

22

Resistance to Rapid Deformation (Impact Resistance) ASTM D2794

23

Customized Testing

In some circumstances, when selecting a coating for a particular application, customized testing may

be requested. Customized testing is defined as tests that, although possibly based on an international

standard, are different in certain ways (for example, increasing the temperature to accelerate corrosion,

performing the test in pressurized environments to increase the pressure and/or increase the

aggressive gases content, or modifying the test solution used). Other customized tests include

attempts to mimic the natural environment where the coatings will be exposed (this environment may

be accelerated using techniques described above with temperature, pressure, or increased

concentration of the aggressive chemical agents). An interesting customized test in pipeline coatings

includes exposure to a moist soil at varying temperatures (where a section of a pipeline is coated and is

subjected to this environment). Continuous monitoring of the Open Circuit Potential (OCP),

Electrochemical Impedance Spectroscopy (EIS) or monitoring the current used to maintain Cathodic

Protection (CP) in the system are some of the techniques that can be employed during customized

testing.

In the results and discussion section of this manuscript, we attempt to present typical data that can be

obtained by using some of these techniques.

4

RESULTS AND DISCUSSION

Testing of Candidate Internal Pipeline Coatings

Mechanical Testing

: In the past few years, a number of coatings (different suppliers and different

materials) have been tested to investigate their potential use as an internal pipeline coating. In this

manuscript, we will show three coatings that were subjected to several corrosion and mechanical tests

(as described in the experimental section). Results of the mechanical testing are presented in Table 3

below.

TABLE 3 Mechanical Testing Of Candidate Internal Pipeline Coatings.

a

Coating

System

Pull Off

Strength

[psi,

(MPa)]

Max Area

of Coating

Substrate

Adhesive

Failure

[%]

Abrasion

Resistance

[mg weight

loss per

1000 cycles]

Bend Test

[2.5

o

Deflection

angle]

Hardness

[Shore D]

Tensile

Strength

[psi,

(MPa)]

Elongation

[%]

Test

Standard ASTM D4541 ASTM D4060

CSA

Z245.20-06

ASTM

D2240 ASTM D638

A

3200

(22.06)

10 25 no break 89

6880

(47.44)

15

B

3400

(23.44

80 23 no break 86

6820

(47.02)

15

E

2660

(18.34)

<5% 7 no break 90

4240

(29.10)

7

a All properties are measured in accordance with applicable test standards; therefore, reported

values are averages of multiple trials.

All coatings considered during this study (A, B and E) showed pull-off adhesion strength ranges typical

for relatively good coating systems (in the 2600–3500 psi [18-24 MPa] range). Coatings A and E

showed primarily cohesive failure between the adhesive and the coating, indicated by the low amount

of coating-substrate adhesive failure. This indicates that the pull off strength measured is limited by the

strength of the adhesive, not by the properties of the coating. Coating B, on the other hand, did show

primarily coating-substrate adhesive failure, indicating that the pull off strength obtained is near the

maximum value for the coating.

All coatings showed good abrasion resistance (less than 25 mg weight loss per 1000 cycles).

However, of all the coatings tested, E had only 7 mg of weight loss per 1000 cycles, which is

significantly less than the other coatings tested.

In addition, the coatings exhibited similar hardness (Shore D) properties (all in the high 80s and up to

90), as expected for these particular coating types.

Most of the coatings tested showed good bend resistance, whereby the coating did not break or

significantly deform when a 2.5° deflection angle was applied. Two coatings that failed the bend test

(data not shown throughout this manuscript) showed adhesive failure at the steel substrate during

testing.

The tensile strength of the three coatings shown in this work showed values well above 4000 psi (27.6

MPa) (typical of 'good' coatings). Furthermore, all three coatings showed 15% or higher elongation.

Coatings A and B had a tensile strength that was approximately 1.6 times more than that found in

Coating E. In addition, Coatings A and B were able to undergo twice the elongation before failing when

compared with Coating E.

5

Based on the mechanical testing alone, it is not relatively easy to rank the performance of the coatings.

Coatings A and B had higher tensile strengths and elongation to failure, but showed less abrasion

resistance than Coating E. Coating B showed less adhesive strength than Coatings A and E. All

coatings; however, performed relatively well across a ll tests, compared with a number of other coating

systems tested (not listed).

It is important to consider end-use application of the coating to aid in determining which coating is best.

In this case, good abrasion resistance may outweigh other performance variables such as elongation to

failure, as the particular application will require resistance to a mild abrasive fluid passing through the

pipeline. Thus, one can make the argument that in this case, Coating E is the best option.

Corrosion and Water Testing

: The three coatings were subjected to selected corrosion and water

testing that included: water vapor transmission, water absorption, hot water immersion, volume

resistivity, and EIS.

All coatings passed the hot water immersion test (this is a 24 hour test at ~65

o

C) and showed good

performances in the water absorption evaluation (at ~38

o

C), with Coating E showing the lowest uptake

(0.006 mg/hr, Table 4) of water. Coating E had water absorption rates that were ~ 14 times lower than

Coatings A and B, indicating that the coating is less likely to uptake water that could lead to the

corrosion of the underlying substrate.

TABLE 4 Corrosion Tests and Water Testing of Candidate Internal Coatings.

a

COMMON COATING 'CORROSION' TEST

Coating

System

Water Vapor

Transmission

b

(mg/hour-cm

2

)

Water Absorption

c

(mg weight gain/hr)

Hot Water

Immersion

d

Volume

Resistivity

(Ohm-cm)

EIS

e

at 0, 1 &

2 months

(Ohms)

Test

Method

ASTM D1653 ASTM D870 NACE TM0174

A 0.03 0.083 1.4 × 10

15

2 × 10

+9

2 × 10

+9

2 × 10

+9

B 0.12 0.083 5.5 × 10

15

6 × 10

+9

6 × 10

+9

4 × 10

+9

E 0.05 0.006

No detrimental

effects observed

8.6 × 10

15

6 × 10

+9

Coating failure

Coating failure

a All properties are measured in accordance with applicable test standards; therefore, reported

values are averages of multiple trials.

b Wet Cup, Method C (Desiccators at 73

o

F [23

o

C]).

c At 38

o

C.

d 150

o

F (65

o

C ) for twenty-four hours.

e Samples were immersed in artificial seawater solution at 60

o

C and the Z (at 10 mHz) was

measured weekly (although data shown is in months to facilitate ease of comparison).

The results from the volume resistivity testing show similar values for all the coatings. The three

coatings also showed properties consistent with those expected for water vapor transmission, with

Coatings A and E performing exceptionally well (0.03 and 0.05 mg/hr• cm

2

, respectively).

6

The results of these tests point toward a conclusion that Coating E is likely the best coating for this

application. Notice that most of these tests were done at relatively low temperature (with the exception

of the twenty-four hour water immersion at ~65

o

C).

The last test that was performed on the three viable candidates was Electrochemical Impedance

Spectroscopy (EIS). The samples were immersed in artificial seawater solution at 60

o

C and the

impedance, Z, (at 10 mHz) was measured weekly (see Table 4). Surprisingly, after less than one month

in the testing environment, Coating E showed disbondment on all the samples. Apparently, the long-

term exposure to the salt solution at the higher temperature has a critical effect on Coating E. On the

other hand, coating A had a stable (and constant) Z value during the first two months of testing. The

value of Z for Coating B, after two months in test, began to decrease.

Notice that Coating E was the leading choice based on the mechanical properties and the water

absorption tests at lower temperature (and even after the high temperature test for a very short period).

However, after less than a month at the higher temperature, and based on the EIS (and visual

confirmation), one can see that Coating E is not a good choice. In fact, the best coating for this

application is Coating A.

Testing of Candidate External Coating System

: Some external coating systems are also being

considered to externally protect the pipeline. At the present time, only one of the coatings has been

received and is currently being tested both using a number of mechanical (see Table 5) and corrosion

tests (see Table 6). Similar to the previous case (internal pipeline coating), the coatings will be

subjected first to mechanical testing and then to corrosion testing.

Mechanical Testing

: Table 5 shows the preliminary data obtained for Coating 1 (of four coatings

currently being considered for this application).

TABLE 5 Mechanical Test Results of Coating 1 (Candidate for External Pipeline Coating).

a

STANDARD TESTING STANDARD RESULT

Abrasion Resistance ASTM D4060 Average Wear Index: 0.01

Adhesion to Steel ASTM D4541 >2500psi (>17 MPa)

Dielectric Strength ASTM D149 1.1 kV/mil (43.3 kV/mm)

Durometer Hardness (Shore D) ASTM D2240 80

Elongation

Tensile Strength

ASTM D638

Strain at break 6.76%

TS: 3297 psi (22.73 MPa)

Flexibility of Coating/Bendability CSA-Z245.20-06 no cracking

Flexibility of Coating/Bendability ASTM D522 no cracking

Lap Shear Adhesion ASTM D1002

Min: 1453 lbf (6.46 kN)

Max: 2489 lbf (11.1 kN)

Mean: 1855 lbf (8.25 kN)

Resistance to Rapid Deformation (Impact Resistance) ASTM D2794 >280 in.-lb.(>104 N)

a All properties measured in accordance with applicable test standards; therefore, reported values

are averages of multiple trials.

7

TABLE 6 Corrosion and Water Test Results of Coating 1, Candidate for External Pipeline Coating.

b

CORROSION TESTING STANDARD RESULT

Porosity, Interface,

and Cross-Sectional

CSA-Z245.20-06

Cross Section Porosity Rating = 1 Interface

Porosity Rating = 3

ASTM G8 Test Ongoing

ASTM G95 Test Ongoing

CSA-Z245.20-06 Test Ongoing

Resistant to

Cathodic Disbondment

ASTM G42 Test Ongoing

Resistant to chemical degradation,

High Temperature

NACE TM0174 Test Ongoing

Water Vapor Transmission ASTM D1653

Avg: 87.12 grams per cm

2

per 24 hrs

(see Figure 1)

Water Absorption ASTM D870

Avg Weight Gain: 5.04%

(See Figure 2)

Salt Fog Testing ASTM B117 Test Ongoing

UV-Condensation ASTM D4587 Color Fade

b All properties measured in accordance with applicable test standards; therefore, reported values

are averages of multiple trials.

The abrasion resistance of Coating 1 shows an Average Wear Index of 0.01, which is similar to that

found in Coatings A, B and E (see previous section on internal coatings). The adhesion to the steel

substrate of this coating shows values below 2500 psi (17.2 MPa), which is lower than those found for

the internal Coatings A and B, and very similar to that obtained for Coating E.

Two tests not considered during the selection procedure for the internal coating (Dielectric Strength and

Durometer Hardness) are important for the qualification of external coatings. The results for Coating 1

show values that are typical for polymeric coatings.

The elongation to failure of Coating 1 is very similar to that of Coating E. The tensile strength of

Coating 1 is not as good when compared to Coatings A, B and E. However, the flexibility and

bendability results are similar to the coatings considered in the previous section.

In general, the mechanical testing of Coating 1 shows relatively good properties compared to the

typical systems that are traditionally considered for these types of applications.

Corrosion and Water Testing

: Table 6 shows the results from the water and corrosion testing for

Coating 1. According to the manufacturer of this coating, this coating has performed exceptionally well

in the mechanical and most of the corrosion testing. However, comparison between the different

coatings using corrosion testing is currently ongoing.

The cross section porosity and interface porosity of Coating 1 show relatively good performance (when

compared with typical systems currently used as external pipeline coatings). Three tests, resistance to

cathodic disbondment, resistance to chemical degradation, and salt fog testing are currently underway.

The water vapor transmission test (WVT), shown in Figure 1, shows triplicate data from Coating 1. The

test was conducted according to ASTM D1653, Test Method B (Wet Cup Method), Condition C (near

zero relative humidity in the surrounding atmosphere) for 1200 hrs. The results show that the steady

water vapor flow in unit time through unit area of the coating induced by unit vapor pressure difference

between the two surfaces of the coating yield values around 3.63 gr/ hour-cm

2

.

8

Water Vapor Transmission Test

0

1

2

3

4

5

0 200 400 600 800 1000

Time (hrs)

Weight Loss (% of original weight)

Sample 1

Sample 2

Sample 3

FIGURE 1 – Water Vapor Transmission Test Performed on Candidates for External Coating.

The water absorption test, shown in Figure 2, was performed on four samples of Coating 1 (tested

according to ASTM D870) at 100

o

F +/- 2 °F (~38

o

C +/- 1 °C) for just over 1000 hrs. No adverse

effects (blistering, damage or discoloration) of the coating samples were observed during or after this

test. All samples showed relatively rapid weight gain in the first 72 hours, then reached a near steady-

state condition with an average weight gain of 5.04% for all the samples.

Water Absorption vs Time

0

0.01

0.02

0.03

0.04

0.05

0 200 400 600 800 1000

Time (hours)

Weight Change (% of original weight)

Sample 1

Sample 2

Sample 3

Sample 4

FIGURE 2 – Water Absorption Test Performed on Candidates for External Coating.

9

An accelerated corrosion test using a miniature flow loop (see Figure 3) is being constructed, using

carbon steel pipes coated with three different coating systems. The main objective of the test is to

evaluate the coatings for a long period of time using electrochemical methods (including EIS) to

investigate the performance of the three external pipeline coatings in simulated real world conditions.

The setup consists of a soil box with representative soil composition and wetted using artificial

seawater. The fluid inside the pipe will be at the same temperature as that which is expected in service

conditions. Moreover, cathodic protection is being applied to the coated pipes to mimic the real

conditions that these coatings would encounter in the field. Evaluations of the current needed to

protect the coating, as well as periodic EIS measurements, are currently underway. These will be

compared with the ACT results when completed.

24

FIGURE 3 - Miniature flow loop for accelerated corrosion testing using EIS

CONCLUSIONS

A number of coatings for both internal and external coating of a pipeline are being considered and are

currently being tested. As expected, no single test method has (or can) accurately predict coating

performance; moreover, numerous combinations of tests can lead to results that are difficult to

interpret. Mechanical tests and accelerated corrosion tests can show varying results, which can

confuse the investigator as to actual expected field performance. This can perhaps be due to ACT not

accurately representing field conditions or accurately mimicking corrosion mechanisms.

It is suggested that a combination of mechanical and corrosion tests be carefully considered when

investigating potential coating systems. The corrosion tests should include real-world conditions (such

as the miniature flow loop in a soil box with accelerated weathering factors and impressed cathodic

protection).

10

REFERENCES

1. ASTM B117-97 – "Standard Practice for Operating Salt Spray (Fog) Apparatus."

2. BSI BS 3900-F4 "Methods of test for paints Part F4: Resistance to continuous salt spray," BS

3900 Part F12 "Paints & Varnishes – Determination of Resistance to neutral salt spray (fog)"

3. ISO 7253:1996 – "Paints and varnishes – Determination of resistance to neutral salt spray (fog)."

4. "Study of Mixed Flowing Gas Exposure of Copper", M. Reid, J. Punch, L. F. Garfias-Mesias, K.

Shannon, S. Belochapkine, and D. A. Tanner, Journal of The Electrochemical Society, 155, (4)

C147-C153 (2008).

5. "Accelerated corrosion tests for aerospace materials: current limitations and future trends,"

Baldwin, K. R. and Smith, C. J. E.

6. ASTM D149-97 – "Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength

of Solid Electrical Insulating Materials at Commercial Power Frequencies."

7. CSA Z245.20-02 – "External Fusion Bond Epoxy Coating for Steel Pipe."

8. ASTM G8-96 – "Standard Test Method for Cathodic Disbonding of Pipeline Coatings."

9. ASTM G42-96 – "Standard Test Method for Cathodic Disbonding of Pipeline Coatings Subjected

to Elevated Temperatures."

10. ASTM G95-87 – "Standard Test Method for Cathodic Disbondment Test of Pipeline Coatings

(Attached Cell Method)."

11. NACE TM0174-2002 – "Standard Test Method: Laboratory Methods for the Evaluation of

Protective Coatings and Lining Materials on Metallic Substrates in Immersion Service."

12. ASTM D4587-01 – "Standard Test Method for Fluorescent UV-Condensation Exposures of Paint

and Related Coatings."

13. ASTM D870-97 – "Standard Test Method for Testing Water Resistance of Coatings Using Water

Immersion."

14. ASTM D1653-93 – "Standard Test Method for Water Vapor Transmission of Organic Coating

Films."

15. ASTM G1-90 – "Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test

Specimens."

16. ASTM D4060-95 – "Standard Test Method for Abrasion Resistance of Organic Coatings by the

Taber Abraser."

17. ASTM D4541-95 – "Standard Test Method for Pull-Off Strength of Coatings Using Portable

Adhesion Testers."

18. ASTM D2240-05 – "Standard Test Method for Rubber Property – Durometer Hardness."

19. ASTM D638-02 – "Standard Test Method for Tensile Properties of Plastics."

20. CSA Z245.20-02 – "External Fusion Bond Epoxy Coating for Steel Pipe."

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21. ASTM D1002 – "Standard Test Method for Apparent Shear Strength of Single-Lap-Joint

Adhesively Bonded Metal Specimens by Tension Loading (Metal-to-Metal).

22. ASTM D522-93a (2008) – "Standard Test Methods for Mandrel Bend Test of Attached Organic

Coatings."

23. ASTM D2794-93 – "Standard Test Method for Resistance of Organic Coatings to the Effects of

Rapid Deformation (Impact)" J. R. Scully and S. T. Hensley, Lifetime Prediction for Organic

Coatings on Steel and Magnesium Alloy Using Electrochemical Impedance Spectroscopy,

Corrosion 50 (9), (1994), p. 705.

24. J. R. Scully and S. T. Hensley, Lifetime Prediction for Organic Coatings on Steel and Magnesium

Alloy Using Electrochemical Impedance Spectroscopy, Corrosion 50 (9), (1994), p. 705.

12

ResearchGate has not been able to resolve any citations for this publication.

This paper describes the results of copper coupons exposed to a class III mixed flowing gas environment (MFG) following the guidelines given by the Battelle Laboratory and the International Electrotechnical Commission for environmental testing. Corrosion products were studied in detail using scanning electron microscope, energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), focused ion beam (FIB), secondary ion mass spectroscopy (SIMS), and transmission electron microscope. The weight gain measured after each exposure was compared with the weight gain calculated from the cathodic reduction of the corrosion layers and cross sectioning using an FIB. The result shows a relatively good correlation between the measured and the calculated experimental values of weight gain. As expected, within the first week, the different corrosion layers thickened until they formed a thick layer that became the determining step for further growth. After several days of exposure the Cu coupons developed a complex multilayered structure consisting of cuprous oxide (Cu2S), cupric oxide (CuO), copper sulfide (Cu2S), covellite (CuS), and evidence of antlerite (3CuO SO3 2H(2)O). No Cl-containing corrosion products were identified using XRD. However, EDS and SIMS analysis showed that Cl was distributed throughout the corrosion products, indicating that although Cl is inside the corrosion products, it is not part of the crystalline structure. Also, this suggests that Cl plays an important role in accelerating the corrosion of Cu during exposure to the MFG class III test. (c) 2008 The Electrochemical Society.

  • J. R. Scully
  • S. T. Hensley

Electrochemical impedance studies were conducted on an epoxy polyamide-coated AISI 1010 steel (UNS G10100) and an epoxy/chromate conversion-coated magnesium (Mg) alloy ZE41A-T5 (UNS M16410). Results were compared to evaluate the general applicability of various impedance-derived measures of coating performance for radially different metal substrates. Both coating systems were immersed in room-temperature aqueous sodium chloride (NaCl) solution of near neutral pH. Correlation of impedance parameters obtained early in exposure with long-term visual appearance demonstrated that low-frequency impedance (< 10 mHz), coating resistance, breakpoint frequency, phase angle minimum forecasted the long0term performance of both systems. Impedance-based parameters adequately estimated the defect areas associated with corrosion sites that penetrated the organic coating but were unable to estimate the area associated with coating blisters. More conservative impedance-based criteria had to be used for the coated Mg alloy than for the coated steel in predicting long-term performance. It was hypothesized that differences in the critical impedance thresholds resulted from rapid Mg oxidation and reduction of water reaction rates that occurred at even the smallest defect sites and from the greater solubility of magnesium hydroxide (Mg[OH][sub 2]). These findings imply that more conservative coating performance criteria than traditionally envisioned for steel in neutral salt water might be necessary for Mg.

  • K.R. Baldwin
  • C.J.E. Smith

Examines accelerated methods for the corrosion testing of materials, coatings and surface treatments used in the aerospace and defence industries. The drawbacks with some current accelerated corrosion tests are examined, particularly the problems experienced with neutral salt spray tests. Specific examples are given which identify the acute discrepancy between salt spray and marine exposure in the corrosion testing of metallic coatings on steels. Examines some recent advances in corrosion testing of aerospace materials, pre-treatments and organic coatings, and outlines some preliminary research conducted at DERA Farnborough in developing more accurate test methods.

Paints and varnishes -Determination of resistance to neutral salt spray (fog)

  • M Reid
  • J Punch
  • L F Garfias-Mesias
  • K Shannon
  • S Belochapkine
  • D A Tanner

ISO 7253:1996 -"Paints and varnishes -Determination of resistance to neutral salt spray (fog)." 4. "Study of Mixed Flowing Gas Exposure of Copper", M. Reid, J. Punch, L. F. Garfias-Mesias, K. Shannon, S. Belochapkine, and D. A. Tanner, Journal of The Electrochemical Society, 155, (4) C147-C153 (2008).