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This paper aims to research the corrosion behavior of the metal under the disbonded coatings interfered with AC through electrochemical method.

The corrosion behavior of the metal under disbond coating interfered with alternate stray current (AC) was studied by electrochemical methods using the rectangular coating disbonded simulator. The obtained data from electrode potential test, electrochemical impedance spectroscopy (EIS) and polarization curves in simulated soil solution indicated that under the natural corrosion condition, the self-corrosion potential and the corrosion current density of the metal at different depths under disbond coating had obviously changed if there was AC interference.

The self-corrosion potential of the metal at the same depths under disbond coating shifted negatively with the rising of the AC voltage. Under the condition of cathode polarization, there was still obvious potential gradient with the extension of the deep peeling of the coating gap, and the corrosion current density of the test points was minimum, and the protection effect was best when the cathode protection potential was −1.0 V. When the metal was applied with over-protection, the corrosion rate of the metal increased as AC stray current flowing through it increased.

This paper used the rectangular aperture device to study the corrosion behavior of X80 steel under the disbonded coatings through electrochemical methods when the AC stray current interference voltage was 0V, 1V, 5V or 10V and the protection potential was 0V, −0.9V, −1.0V, −1.2V or −1.3V, respectively. There is great significance to the safe operation and long-term service of pipeline steel in soil environment.

Introduction It is well appreciated that the primary purpose of an external coating on a pipeline is of course to prevent corrosion and the coating system should possess appropriate physical and chemical properties which allow this function to be fulfilled. There are, however, additional mechanical requirements of a coating system if the material is to be effective during handling, construction, testing and operation and it is within the context of these latter requirements that current interest in external coatings has arisen.

Pipelines are important and expensive pieces of capital equipment and their owners are interested in their safe and economic operation. This requires the reliable protection of the pipes against corrosion by the use of external coatings and/or cathodic protection.

In this investigation, attempts were made to distinguish critical coating faults in order of repair priority in DCVG+CIPS survey data by new proposed equation for calculating IR drop. The paper aims to discuss these issues.

DCVG+CIPS, XRD, EDS.

A new empirical equation was developed which is able to predict the importance more precisely. Besides, the studies proved the corrosion products were mainly hematite and goethite, and the presence of Cl anions was not noticed. As a result, the corrosion attack had a form of non-uniform localized corrosion.

The values were gathered by DCVG+CIPS method of survey.

The most economical way of protecting pipelines is through the application of organic coatings and wrappings. To make the protection foolproof at holidays and pores, cathodic protection is also applied. At coating holidays, the pipelines act as the cathode generating hydroxyl ion and hydrogen electrochemically. Extensive studies have been carried out using samples of mild steel pipe coated with reinforced coal tar enamel coating to investigate the role of hydrogen and hydroxyl ion on cathodic disbondment. The testing material is subjected to different environmental conditions like pH, aeration and addition of different anionic species. A mechanism has been proposed to describe the individual role of hydroxyl ion and hydrogen in the process of disbondment.

This paper aims to make clear the sensitive zone of subsea pipeline to stress corrosion cracking (SCC) under a disbonded coating.

The change of microenvironment under a disbonded coating in artificial seawater was analyzed by using a rectangular crevice cell. The SCC behavior of subsea pipeline was studied by slow strain rate tensile tests.

The microenvironment at the crevice bottom exhibits obvious acidification, Cl- aggregation and cathodic protection potential (CP) rise. Accordingly, the susceptibility of X70 steels to SCC is high due to the intensive anodic dissolution effect. At the opening, hydrogen atom can access into the steel and induce hydrogen embrittlement effect on account of the applied over-protected CP potential, resulting in a relatively high susceptibility to SCC. The corrosiveness of the microenvironment at crevice middle, however, is mild with proper CP potential; thus, the susceptibility of X70 steel to SCC here is lower than that obtained at the opening and the crevice bottom.

A rectangular crevice cell is built to survey the microenvironment evolution under a disbonded coating in situ. The sensitive zone of subsea pipeline to SCC under a disbonded coating is clarified.

A typical coated system comprises a substrate to which a coating has been applied. The coating in turn usually consists of several layers. Failure of the system may result either cohesively or adhesively. Cohesive failure may cause the substrate itself to separate and thus the system has been injured. Although cohesive failure of the substrate is usually not a problem with metal, it may well be a problem with concrete. And, indeed, it may be facilitated if the coating adheres so strongly that forces at the coating‐concrete interface set up stresses within the concrete which cause it to disbond cohesively. Of course cohesive failure may occur within the coating itself or, if the layers of the coating are sufficiently distinct, within any one layer of the coating system.

The disbonding of DLC coating is a main failure mode in the high-speed cavitation condition, which shortens the service life of the bearing. This study aims to investigate influence of adhesion strength on cavitation erosion resistance of DLC coating.

Three DLC coatings with different adhesion strengths were grown on the 304 steel surfaces by using a cathodic arc plasma deposition method. Cavitation tests were performed by using a vibratory test rig to investigate the influence of adhesion strength on cavitation erosion resistance of a DLC coating. The cavitation mechanism of the substrate-coating systems was further discussed by means of surface analyses.

The results indicated that, the residual stress decreased and then increased with the increasing DLC coating thickness from 1 µm to 2.9 µm, and the lower residual stress can improve the adhesion strength of the DLC coating to the substrate. It was also concluded that, the plastic deformation as well as the fracture occurred on the DLC coating surface at the same time, owing to higher residual stress and poorer adhesion strength. However, lower residual stress and better adhesion strength could help resist the occurrence of the coating fracture.

Cavitation tests were performed by using a vibratory test rig to investigate the influence of adhesion strength on cavitation erosion resistance of the DLC coating. The plastic deformation and the fracture occurred on the DLC coating surface at the same time, owing to higher residual stress and poorer adhesion of coating. Lower residual stress and better adhesion of coating could resist the occurrence of the DLC coating fracture.

The development of the Hutton Field in the UK sector of the North Sea incorporates, for the first time, a buoyant Tension Leg Platform maintained in position by an array of sixteen tubular steel tension leg strings. To satisfy the stringent design criteria associated with this new generation of offshore oil production facilities, all corrosion control systems have to be effective, reliable and contribute little to structure weight. An intensive review of the various available design options was undertaken, from which emerged the use of aluminium metal sprayed coatings as possibly the optimum method of corrosion control for the tension legs. Since there was limited service data available relating to the performance of sprayed metal coatings in sea water immersion service, a development programme was completed to determine the perfomance characteristics of these coatings under tension leg operating conditions. In parallel, the problems of applying the coatings were addressed since it was considered that, as for all coating systems, correct application is critical to the satisfactory in‐service performance of the coating. The potential advantages of sprayed metal coatings for immersion service in the offshore industry was significant.

This article is presented as a summary of the authors' experience, over the last decade, of the use of cold applied self‐adhesive pipewraps for the refurbishment, in situ, of operational pipelines where coating failures had occurred and other similar occasions when the coating application had to be carried out on an excavated but operational pipeline.