Corrosion and Cathodic Protection at Disbonded Coatingsproceedings.asmedigitalcollection.asme.org/data/... · CORROSION AND CATHODIC PROTECTION AT DISBONDED COATINGS J.H. Payer, K.M

In ternational Pipeline Conference — V o lu m e 1A S M E 1996

CORROSION AND CATHODIC PROTECTION AT DISBONDED COATINGS

J.H. Payer, K.M. Fink, J .J . Perdomo, R.E. Rodriguez, I. Song and B. Trautm an

Department of Materials Science and Engineering Case Western Reserve University

10900 Euclid Avenue Cleveland, Ohio 44106

ABSTRACT

The effectiveness of cathodic protection to control corrosion and the resulting corrosion rate of pipelines arc determined by the chemical and electrochemical conditions at local areas along the pipeline. The disbonding of coatings and tapes is also controlled to a large extent by the chemical and electrochemical conditions. Processes that occur on the metal surface and their effect on corrosion and cathodic protection are discussed with respect to real pipeline conditions. Disbonded coatings on steel can interfere with the current distribution from cathodic protection. Shielding the current under disbonded coatings can affect the level of protection, the corrosion behavior and the disbonding of coatings. A major thrust in our laboratories has been the use of laboratory measurements and computational models to determine the changes in the corrosive environment that occur beneath disbonded coatings as a function of applied potential, disbonded area geometry, prior corrosion products and wet/dry cycles. These results are summarized here.

INTRODUCTION

Buried pipelines are protected from corrosion by a combination of a protective organic coating and cathodic protection. The cathodic protection system is designed to protect steel where the coating is damaged. Aging pipelines can have degraded coatings and sites of corrosion. A major issue facing the industry today is the assessment of current status and reliability of aging pipelines.

Evaluation of pipelines requires a combination of operating experience, inspection technology, corrosion control technology-coatings and cathodic protection, and reliability/life prediction technology. The inspection technology to locate and measure corrosion damage is improving. Improved empirical and deterministic reliability methods for pipelines are required.

A deterministic approach to pipeline integrity analysis requires knowledge of the potential and solution chemistry of steel at exposed areas. Significant progress has been made in this area over the last several years; however, there is much work to be done to permit predictions on a sound engineering basis.

A major thrust of our present work is to extend the deterministic understanding to include a broader range of relevant pipeline operating conditions. There has only been limited data for the effects of prior corrosion products in the exposed steel area and the effects of cyclic environmental conditions, e.g., wet/dry cycles and interruptions to cathodic current flow to the exposed steel. The interactions between coatings degradation, e.g., disbonding, and cathodic protection are not clearly understood.

In this paper, the chemical and electrochemical processes that occur at the pipeline steel surface are identified. Recent work on factors controlling the cathodic disbonding process are described. Results to determine the potential and solution conditions under disbonded coatings are summarized.

CHEMICAL AND ELECTROCHEMICAL PR O CESSES

The corrosion behavior of steel is commonly described by the chemical composition of an aqueous solution in contact with the steel surface and the electrochemical potential of the steel. This is shown schematically in the figures below which represent steel exposed to ground waters at a holiday in the protective coating on a pipeline. Based upon the solution composition and the potential of the steel in the solution, the steel will exhibit immune, passive or active behavior. If the steel is polarized to highly reducing potentials, the steel is thermodynamically stable and will not corrode. If the steel is exposed to potential (E) and pH combinations in the passive

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range, the steel will not continue to conrode because the corrosion process forms a protective layer of insoluble corrosion products. If the potential and pH combinations are in regions where soluble corrosion products are stable, the steel will continue to corrode.

The corrosion rate of steel is determined by the combined anodic and cathodic polarization behavior on the steel in the holiday and beneath any disbonded coating. The solution composition and potential will determine the combined effects of activation, concentration and ohmic polarizations. There are microcell and macrocell effects within the region due to current distribution throughout the disbonded area.

The pipeline steel can exhibit three modes of behavior shown in Fig. 1. The steel will be immune (no corrosion), passive (protected by an insoluble film) or active (corrosion). The corrosion proceeds when the corrosion products are soluble or when the insoluble products are non-protective. Which of the three modes of behavior will apply will depend upon the composition of the solution in contact with the steel and the potential of the steel surface. The potential-pH diagram for iron identifies the relative regions over which each behavior will be observed in pure water (Pourbaix, 1974). The presence of other ionic species in the water can significantly affect the location of the corrosion, immune and passive regions on the diagram.

Figure 2 illustrates how electrochemical processes in a given chemical environment can affect the integrity of the pipeline protection system. Failure at any interface or in any of the interfacial regions of metal, oxide or coating will result in loss of adhesion and disbondment of the protective coating. It is important to recognize that a variety of processes contribute to the behavior and resulting durability of each of these areas. The effectiveness, and thus the usefulness, of the coating will be determined by the weakest region, and useful life will be extended only by strengthening the weakest region. Depending upon the exposure conditions, the controlling mechanism can change from failure at one interface to failure at another. It is not surprising, then, that a variety and apparently conflicting set of observations and conclusions have been reported from laboratory and field studies of cathodic disbonding. In Figs. 2 and 3, the processes that affect the behavior at each interface in the metal/coating system are described.

The electrochemical polarization processes on steel at a holiday or within a disbonded area are shown in Fig. 3. These polarization processes describe the rate of cathodic and anodic reactions and are determined by the inherent reactivity (activation polarization), concentration and transport of reaction species (concentration polarization) and electronic or ionic conductivity (ohmic polarization). Each type of polarization will depend upon the solution composition and potential of the steel. Important effects occur over large areas (macrocells) or in local regions (microcells). The access to and distribution of cathodic current over the disbonded area depends on the magnitude of current and the geometry of the disbonded area.

fSTEEL BEHAVIOR

A

■ Im m u n e - I r o n is s ta b le , n o re a c tio n

•P a s s iv e - In s o lu b le , P ro te c t iv e F i lm ;

o x id e , F e jO ^ F e O O H ,

p h o s p h a te , c h ro m a te . . .

• C o r r o s io n - S o lu b le C o r r o s io n P r o d u c t

A c id — F e++

A lk a l in e — H F e 0 2 *

- N o n - P r o te c t iv e O x id e : F e ^ O j

C a th o d icP ro te c tio n

C u rre n t

Fig. 1. Corrosion behavior of steel.

The two principal cathodic reactions on buried steel structures are oxygen reduction and hydrogen evolution. Both of these reactions result in the solution at the holiday becoming more alkaline (higher pH). An additional consideration identified in the work at Case is the damage that can result from other products generated during cathodic polarization, e.g., peroxide and similar species (Gervasio and Payer, 1992; Payer et al., 1993a-c). The detrimental effects of alkaline solutions on organic coatings is well recognized, but the generation of higher alkalinity solutions at the holiday also results in a beneficial effect. The benefit is that steel corrosion is greatly reduced in alkaline solutions (Payer et al.. 1994; Perdomo and Payer,1995).

The reactions that occur and the rates of those reactions will depend upon the transport processes into the holiday and along the disbonded region. The transport of gases, water and ionic species are important in the overall process. Tire


transport will be controlled by the geometry of the disbonded area, concentration gradients, potential gradients and convection. The effects of wetVdry cycles are an area that is clearly important and yet an area that has received little serious study.

f -------------------------------------------- \POSSIBLE DISBONDING PROCESSES

• M e ta l o x id e d is s o lu t io n

• I n te r fa c ia l p o ly m e r a t ta c k

• P o l y m e r / o x id e a d h e s io n lo ss

• M e ta l d is s o lu t io n

• M e t a l / m e t a l o x id e a d h e s io n lo s s

v______________ _______________ /

H o l id a y

•Steel Substrate::

Metal Oxide

¡Metal Substrate':

F B E

CathodicDisbonding

• Polymer chemicalresistance

• Oxide stability• Solution composition• Potential

C a th o d ic

P ro te c tio n

C u rre n t

Soil

Fig. 2. Possible disbonding processes.

CATHODIC DISBONDING PROCESS

Organic coatings are all permeable to water, gases and ions to varying degrees. They are not true barrier films. The permeabilities will depend upon coating composition and thickness. Some coatings will transport sufficient water and oxygen to support corrosion reactions beneath an intact coating. This leads ultimately to coating disbonding and failure. The chemical stability and adhesion of the coating to the pipeline is affected by its formulation. The surface preparation and coating process are critical to behavior and performance.

Disbonding of coatings and pipeline corrosion have been examined in our laboratories for several years (Gervasio et al..

1992, 1993; Trautman, 1994; Rodriguez, 1996). The extent of cathodic disbonding is strongly dependent upon the chemical and electrochemical conditions. Selected results from our work are shown in Figs. 4-6. The important factors of starting solution composition, level of cathodic protection, level of aeration and wet/dry cycles greatly affect the rate of disbonding and the total amount of disbonding. None of these factors are addressed in standard cathodic disbondment tests, and this results in poor correlation between the standard tests and field performance.

---------------------------------------------NPOLARIZATION PROCESSES

•Activation: A nodic/O xidationCa thodic/Reduc tion

•Concentration: Oxygen

•O hm ic (iR): Soil, Solution, Films

\_________________________ /

C a th o d ic

P ro te c t io n

C u r re n t

Fig. 3. Electrochemical polarization processes.

Figure 4 demonstrates the effect of starting solution chemistry and level of cathodic protection on the extent of disbonding. Starting tests with a strongly alkaline solution resulted in more severe disbonding, and disbonding was much more severe as the level of cathodic protection was increased in all three starting solutions. The level of aeration was also found


to have a major affect oil caihodic disbonding. Starting with a neutral environment and the same level of cathodic protection, over a 4 month period, the extent of disbonding was 25 times more severe for an air-saturated solution (25 cm2 disbonded area) than for a deaerated solution (less than 1 cm2).

Fig. 4. Disbonding distance as a function of starting solution and level of cathodic protection.

In a standard cathodic disbonding test, the positioning of the anode during the test was shown to have a great effect on the amount of disbonding for an FBE (Fusion Bonded Epoxy) coating. When the anode was in the same test compartment as the cathodic disbonding specimens, rapid and extensive disbonding occurred. When the anode was placed in a separate compartment, the extent of disbonding was greatly reduced. These standard tests were run in an aqueous chloride solution, and gaseous chlorine was generated at the anode throughout the test. If this chlorine can migrate to the specimens, accelerated disbonding is observed. The effect of this was also observed in the "bleaching'“ of color from the specimen coatings. While this condition accelerates disbonding in the laboratory, it is not relevant to field conditions. Yet, cathodic disbonding tests are still run by some using this test procedure.

Figures 5 and 6 show the dramatic increase in disbonding with wet/dry cycles compared to that observed for continuously wet conditions. Under most field conditions, alternate wetting and drying cycles are experienced with time, and none of the standard cathodic disbonding test protocols address this effect.

C O N D IT IO N S U N D E R D IS B O N D E D C O A T IN G S

Potential and solution chemistry have been experimentally measured to simulate the effects of highly insulating disbonded coatings on buried pipelines under cathodic protection (Perdomo et al., 1996a; Perdomo and Payer, 1996). Figure 7 shows the potentials in various locations within a disbonded coating in laboratory test cells. Potentials within a disbondmeni shift from a protection range (i.e., more cathodic than -0.85 V vs. Cu/CuS04, CCS) to a corrosion range (i.e., toward the corrosion potential of steel) after each of the five drying cycles. When rewetting occurs, the potential shifts back to the protection range as long as CP (Cathodic Protection) is maintained. Figure 8 shows the effect of caihodic protection interruption on solution pH. After short limes (hours) the pH rises to levels (pH-11) where corrosion of steel is expected to be low. When the CP current is interrupted, the pH shifts to lower levels and corrosion occurs, but when CP is reestablished by rewetting, the protective pH is re-established and corrosion is controlled by the change of the environment.

Fig. 5. Comparison of disbonding rates for specimens exposed to continuous wetting and cyclic wetting-and-drying.

Recent work (Perdomo et al., 1996a; Perdomo and Payer.1996) has suggested that somewhat caihodic potential values can be obtained even in the absence of current as long as the environment directly in contact with the steel surface is alkaline (pH>9) and deaerated. This has been corroborated by modeling the changes within a disbonded coating crevice solution (Perdomo et al., 1996b). The model used a one dimensional (length > width > thickness) crevice where no current tlow was assumed to occur directly through the coating. Even though the current flow to occluded areas will depend on tire resistivity of the medium and thickness of the disbondments. chemical changes (high pH and low oxygen) can be provided in occluded areas (where conditions of stagnation prevail) by diffusion and migration of species created by both the concentration and


potential gradients.

Another important issue arisen from these results (Perdomo et al., 1996a; Perdomo and Payer, 1996) is that the surfaces of the steel pipes that have been exposed to real environments are not in bare metallic conditions but have oxides such as magnetite on the surfaces. These oxide films will thus affect the corrosion rate of the pipe. Previous work (Perdomo et al., 1996a,b; Perdomo and Payer, 1996; Whitman et al., 1923, 1924) has looked at the determination of corrosion rates using "fresh” steel samples where the presence of previous corrosion products has been ignored. Because the kinetics of formation and transformation of oxide films depends strongly on the characteristics of the steel surface and the environment, the electrochemical behavior of the steel surface with pre-existing oxides (e.g., magnetite) needs to be studied. This will also help to clarify the uncertainties in our current understanding of the oxidation scheme of magnetite to FeJ+ species, as to whether the Fe3+ species will be soluble or insoluble in a given environment (Evans, 1960, 1969; Evans and Taylor, 1972). Corrosion rates determined from a specimen with an oxide of interest (e.g., magnetite) on the surface could be more realistic in terms of predicting the useful, remaining life of existing pipelines.

6 .0 -

S 4 .0

Dry Dry

Dry

DryDry

RewetRewet

V

Rewet

Rewet

y y/y

// Wet/Dry

Continuous wetting-

0.0 *£■------1-------- i--------i-------- i-------- i-------- U J____ l_

2.0

coating as well as through macroscopic defects, e.g., holidays or macrocracks. Furthermore, these microcracks allow less restricted transport of species to and from the steel surface. Since the microcracks allow current to be easily passed, the environment at the steel surface can be readily modified to an alkaline condition where corrosion is minimized. However, interruption of CP could result in a more rapid loss of this chemical protection to the surroundings than with an impermeable coating because of the microcracks.

on(JO

-0.60

-0.70

-0.80

-0.90

- 1.00

- 1.10

- 1.20

-1.30

-1.400 5 10 15 20 25

Days

Fig. 7. Effect of cathodic protection interruption on potential of steel surface along a simulated disbonded cell in 10 mM Na2S 0 4. CP at -1.08 V vs. CCS in a 0.89 mm (35 mil) crevice. Arrows indicate the points of drying.

0 4 6 12 16 20Time (days)

Fig. 6. Disbonding distance for wet/dry and continuously wet conditions

As pipelines age, microcracks may develop and the coatings may become permeable. Therefore, it is necessary to design laboratory tests to simulate the conditions of such existing, aging pipelines and the effects of CP on these pipelines. This is an important consideration because the CP current can reach the steel surface through microcracks in the

Determination of the chemical composition and structure of corrosion products after controlled exposures provides insight into the chemical and electrochemical processes underway within corroding regions. An array of microscopies and spectroscopies are used to examine the corrosion products and deposits from the laboratory exposures. Comparisons of these results with existing knowledge regarding the transformation and growth of oxides and other corrosion products can be used to develop protocols to determine conditions of active and inactive corrosion. One such scheme for the structural transitions in iron oxide/hydroxide is given by Bernal et al. (1959). For example, in their report, transitions from hydrated,


ferrous to anhydrous, alpha Fe203 are shown as a function of oxidation and dehydration processes. These processes can be used to rationalize the effects of wet/dry cycles and changing levels of polarization on corroding areas along pipelines.

0 5 1 0 1 5 2 0 2 5

Days

Fig. 8. Effect of cathodic protection interruption on pH of the crevice solution along a simulated disbonded cell in 10 mM Na-,S04. CP at -1.08 V vs. CCS in a 0.89 mm (35 mil) crevice. Arrows indicate the points of drying.

SU M M A RY

The effectiveness of cathodic protection to control corrosion and the resulting corrosion rate of pipelines are determined by the chemical and electrochemical conditions at local areas along the pipeline. The condition of protective coatings and tapes on the pipeline greatly influences the response to cathodic protection and effectiveness of corrosion control. The results of an on-going laboratory-based program to better understand these processes have been summarized here. The results are relevant to the following important questions for operating pipelines:

• Is the corrosion active now?• When did the corrosion occur?• What is the corrosion rate if the pipeline continues to

operate as is?

• Is the present cathodic protection system giving effective corrosion control?

• Were prior cathodic protection upgrades effective in controlling corrosion?

• What will be the benefit of upgraded cathodic protection on corrosion rate?

• How does the current condition of the coating influence cathodic protection?

• Does the cathodic protection lead to further coating degradation?

• Is rehabilitation necessary?

A deterministic approach to pipeline integrity analysis requires knowledge of the potential and solution chemistry of steel at exposed areas. Significant progress has been made in this area over the last several years; however, there is much work to be done to permit predictions on a sound engineering basis. A major thrust of present work is to extend the deterministic understanding to include a broader range of relevant pipeline operating conditions.

ACKNO W LEDGM EN TS

Work in this area at Case has been supported from several sources: (a) the Gas Research Institute; (b) the Alyeska Pipeline Service Co.; and (c) Pipeline Research Committee of American Gas Association. The authors also wish to acknowledge 3M Austin Center for donating the epoxy resins.

REFERENCES

Bernal, J.D., Dasgupta, D.R., and MacKay, A.L., 1959, “The oxides and hydroxides of iron and their structural interrelationships,” Clay Miner. Bull. 4, 15.

Evans, U.R., 1960, "The economic advantages of a sound painting scheme," Trans. Inst. Metal Finish. 37, 1.

Evans, U.R., 1969, "Mechanism of rusting", Corr. Sci., 9, 813.

Evans, U.R., and Taylor, C.A., 1972, "Mechanism of atmospheric rusting", Corr. Sci. 12, 227.

Gervasio, D„ and Payer, J.H., 1992, “Cyclic potential ring measurements (CPRM) for studying oxygen reduction on steel in basic aqueous solution with relevance to disbonding of coatings from cathodically protected steel,” Abstract No. 128, Extended Abstracts of 182nd Meeting of the Electrochemical Society in Toronto. The Electrochemical Society, Pennington, NJ.

Gervasio, D., Song, I., Trautman, B., and Payer, J.H.,1992, “Fundamental research on disbonding of pipelinecoatings,” GRI Report No. GRI-92/0166, Gas ResearchInstitute, Chicago IL.

Gervasio, D„ Song, I., Trautman, B., and Payer, J.H.,1993, “Fundamental research on disbonding of pipelinecoatings,” GRI Report No. GRI-93/0265, Gas ResearchInstitute, Chicago, IL.


Payer, J.H., Trautman, B„ and Gervasio, D„ 1993a, “Methods to determine the role of electrochemical reduction products on coating disbonding from cathodically protected steel,” pp. 109-110, in Proc. of the Am. Chem. Soc. Spring Mtg.. Denver. CO. Div. of Polymeric Materials: Science and Engineering. Vol. 68, American Chemical Society. .

Payer, J.H., Trautman, B., Gervasio, D„ and Song, I„ 1993b, “Role of coating/oxide/steel interfaces on cathodic disbonding of pipeline coatings,” pp. 21-26, Mat. Res. Soc. Svmp. Proc.. Vol. 304, Polymer/Inorganic Interfaces, Opila, R.L., Boerio, F.J., and Czandema, A.W., Eds., Materials Research Society, Pittsburgh.

Payer, J.H., Trautman, B„ and Gervasio,. D„ 1993c, “Chemical and Electrochemical processes of cathodic disbonding of pipeline coating,” Paper No. 579, CORROSION 93. NACE annual conference in New Orleans, National Association of Corrosion Engineers, Houston, TX.

Payer, J.H., Trautman, B., Fink, K„ and Song, I., 1994, “Mechanism of cathodic disbonding of pipeline coatings,” presented at 6th annual Pipeline Monitoring and Rehabilitation Seminar. Houston, TX.

Perdomo, J.J., and Payer, J.H., 1995, “Chemical and electrochemical conditions on steel at disbonded coatings,” AGA/PRC Report No. PR75-9310.

Perdomo-Diaz, J.J., Chabica, M.E., and Payer, J.H., 1996a, "Chemical and electrochemical conditions on steel under disbonded coatings: The effect of applied potential, solution resistivity, crevice thickness and holiday size", to be submitted to Corrosion.

Perdomo-Diaz, J.J., Cawley, J.D., and Payer, J.H., 1996b, "Chemical and electrochemical conditions on steel under disbonded coatings: Mathematical modeling of mass transport and chemical changes in crevices", to be submitted to Corrosion.

Perdomo-Diaz, J.J., and Payer, J.H., 1996, "Chemical and electrochemical conditions on steel under disbonded coatings: The effect of previously corroded surfaces and wet and dry cycles", to be submitted to Corrosion.

Pourbaix, M„ 1974, Atlas of electrochemical equilibria in aqueous solutions, pp. 307-321, NACE, Cebelcor, Brussels.

Rodriguez, R.E., 1996, “Influencing factors in cathodic disbonding of fusion bonded epoxy coatings from a steel substrate,” M.S. Thesis, Case Western Reserve University, Cleveland, Ohio.

Trautman, B.L., 1994, “Cathodic disbonding of fusion bonded epoxy coatings,” M.S. Thesis, Case Western Reserve University, Cleveland, Ohio.

Whitman, W„ Russel, R., Welling, C., Cochrane, J., 1923, "The effect of velocity on the corrosion of steel in sulfuric acid,” Ind. Eng. Chem. 15, 672.

Whitman, W., Russel, R., and Aliteri, V., 1924, "Effect of hydrogen-ion concentration on the submerged corrosion of steel", Ind.. Eng. Chem. 16, 665.


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Corrosion and Cathodic Protection at Disbonded Coatingsproceedings.asmedigitalcollection.asme.org/data/... · CORROSION AND CATHODIC PROTECTION AT DISBONDED COATINGS J.H. Payer, K.M