CORROSION AND CORROSION PROTECTION - RECENT ACTIVITIES
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Coating degradation and long-term performance
For existing underground pipelines it is important to understand the degradation mechanisms and how they impact on the coating's performance during 50 years or more service life. For the coating systems employed by EPRG members, the main causes of premature degradation are found to be an inappropriate specification, inadequate control of metal surface cleaning and preparation, high operating temperatures, and excessive loading due to the surrounding soil. Areas identified for further research include the long-term adhesion of coatings, and the relationship between coating condition and CP shielding; an ongoing project is addressing these issues.
External stress corrosion cracking (SCC)
External SCC has been an issue for pipeline operators since the first in-service failures occurred in the US in the mid-1960s, although it has been extremely rare in Europe. EPRG's activities have concentrated on exploring the relationship between steel type (strength and microstructure) and the likelihood of near-neutral pH SCC developing. The results showed that steels with ferrite-pearlite banded microstructures were more prone to crack initiation than a steel with a fine ferrite-bainite microstructure, and that different inclusion distributions in the steels did not have a significant influence; however, all the steels showed similar crack propagation characteristics. The results suggest that for existing pipelines that already have SCC, reducing the operating pressure or pressure fluctuations may arrest shallower cracks. For new pipeline construction, selecting steels with fine, uniform, initiation-resistant microstructures may be preferable.
Dynamic threshold for Stress Corrosion Cracking in several line pipe steels
Hydrogen-induced Cracking (HIC)
Wet sour gas can be extremely corrosive to low alloy steel pipelines, resulting in blister formation and cracking mechanisms such as hydrogen-induced cracking (HIC). EPRG recently commissioned an experimental programme of HIC exposure tests on different steel types, to provide the basis for developing a preliminary ‘regions of environmental severity’ diagram, similar to that developed for sulphide stress corrosion cracking. The diagram helps in defining the HIC sour service limits for linepipe steels. A further programme of testing has been initiated to substantiate and confirm the preliminary results.
Proposed 'map' of HIC severity regions
Assessing long-term resistance to coating adhesion loss
A review of existing international standards showed that there is no reliable test method for evaluating the long-term service performance of pipe coatings. EPRG undertook a programme to develop a suitable accelerated test method, focusing on the known disbonding mechanisms in 3-layer polyolefin and epoxy pipe coatings; water permeation through the intact topcoat, cathodic disbonding at coating defects and water penetration into the primer layer. The main objective of the work was to find the basis for an accelerated test; exposure for 28 days to water vapour at 60ºC was eventually selected. Further work will extend the range of coating types and pre-treatments, and will correlate test behaviour with long-term service exposure.
Results of accelerated water vapour test on polyolefin pipe coatings
Hostile environmental effects on the residual strength of mechanically damaged pipe
Following a review of service experience that confirmed the contribution of the environment to mechanical damage severity in several service failures, EPRG commissioned an experimental project to explore whether the effect can be simulated and measured in laboratory tests. The work utilised test specimens cut from the wall of pipes containing artificial gouge and dent+gouge damage. The results qualitatively confirmed the effects seen in service, and highlighted the wide range of complex interactions between the pipe material, the damage and the environment. The results have also pointed to the need for testing full pipe sections rather than laboratory specimens, and a programme of full-scale testing has been initiated.
Extraction of test sample containing pipe damage
Cracks in the gouge area after the environment test
Hydrogen-induced cracking (HIC)
Wet sour gas can be extremely corrosive to low alloy steel pipelines. EPRG's recent activities have focused on a review of the testing methods for determining HIC resistance, showing that the NACE test provides a sound basis for selecting and evaluating steels for sour service.
The remaining life of mechanically damaged pipes when exposed to the external environment
As part of a broader programme concerned with the structural integrity of mechanically damaged pipe, EPRG commissioned a review to examine whether the cathodic protection process could promote hydrogen-assisted cracking in the damaged region. It appeared that failures could occur in all grades of steel, subjected to varying degrees of mechanical damage, if the necessary environmental and loading conditions were met. Pressure fluctuations, particularly those at low frequency, increased susceptibility to this failure mechanism.