Technical Presentations at the April 2004 Meeting

1.1         ‘Assessment of Corrosion and Cracking Susceptibility of Welds’, Alan Turnbull (NPL) 

An overview was given of work at NPL over the last few years in relation to the development of new or improved test methods for assessing the resistance of welds to corrosion and stress corrosion cracking. A starting point for that work was a major review of corrosion testing of welds undertaken in 1999 but available for general release only recently. A copy can be downloaded from:

http://www.npl.co.uk/npl/cmmt/aqueous/oil_and_gas/downloads/cmmt_a_130_corrosion_testing_of_welds.pdf 

The three major areas investigated included detection of alloy depletion associated with sigma phase formation in a super-duplex stainless steel (SDSS); preferential corrosion of carbon steel welds, and a more recent investigation looking at factors affecting stress corrosion testing of super 13 Cr steel welds. 

SDSS (Corrosion, 58, 2002, 1039). 

The primary objective was to develop a method that would be sensitive to the Cr dealloyed zone associated with formation of FeCrMo intermetallic and would indicate whether sustained propagation was likely. It also had to be simple to use, including field application. The critical pitting temperature test (CPT) did not satisfy the latter (though examined for comparison) but also could not distinguish welds with different heat input or indeed a failed weld. Essentially, this is because the method responds to localised attack from any source. Also, the CPT can be determined by a single active site and thus does not reflect the extent of overall activity. In principle, the electropotentiodynamic reactivation ( EPR) technique should be better and indeed was more effective but also struggled because the region of sigma phase formation was highly localised in the HAZ. Accordingly, using a probe with 1 cm diameter exposed area, the effect of the sigma phase region on the charge ratio was small and discrimination between different welds was difficult. In principle, smaller probes could be developed but it would be necessary to step along the surface and make a series of measurements. The galvanostatic method developed at NPL simply involves application of a constant anodic current (100 mAcm-2) and monitoring the potential for one hour. If there is no significant depletion the potential increases to the region of oxidation of chloride ions or hydroxide ions and stays nearly constant. If there is a sensitised region, the potential rises initially but following initiation of localised attack, the potential decreases, often with noise indicating new pits activating. The sustained fall, rather than recovery, is indicative that pits would continue to propagate. The method worked well for the controlled welds and was able to demonstrate sensitivity of the failed weld from service. 

Carbon steel welds (Br. Corros. J., 37, 2002, 182)

Here, the challenge was similar but focused on preferential corrosion of the weld or HAZ in seawater. The galvanostatic method was adapted for this, the difference being that visual examination was the only requirement. The principle is elementary. In response to the application of an anodic current, the material will yield the most susceptible regions preferentially. Visual inspection is then used to detect regions of local mass loss. Depending on the current the test could last from a few hours to a couple of days. Too aggressive a test could lead to such general corrosion that discrimination is not possible but 88 mA cm-2 for 2 hours was adequate to reproduce a service weld failure.

Super 13 Cr martensitic stainless steel welds.

This work is ongoing and involves testing of welds in internal fluid associated with oil and gas pipeline as well as cathodically protected steel. The key issue is how best to test and, in the latter, what constitutes a satisfactory qualification criterion. We have undertaken miniaturised tensile testing of samples taken from the weld metal, HAZ and parent plate at temperatures up to 130 °C. The data show a marked decrease in yield strength of the SDSS filler with temperature, which raises the possibility (allied with creep) of significant stress relaxation during 4 pt bend testing. Further work is ongoing using FEA with these input tensile data and strain gauge specimens to assess just how significant this is. 

A parallel issue is the variability in through-thickness properties of a weld. Using a novel scanning microhardness technique, it is possible to readily discriminate local hardness regions; for example,e in the base of the weld root at the second pass interface, as shown below. Note that the black spots here are just artefacts from converting from the colour picture.  The implication of such hardness variability arises when testing specimens machined from the weld that are thinner than full-wall thickness; the specimens may then not be representative.  Interestingly, the hard zones corresponded to regions of compressive stress. It raises the more general question of the uncertainty in hardness in welds when tensile or compressive residual stresses are present.

   

Microhardness scan of super 13 Cr weld with super-duplex filler

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1.2  ‘Internet-based Corrosion Information Systems’, Bob Cottis (Corrosion and Protection Centre, UMIST) 

Corrosion information on the web can be defined in general categories of general corrosion information sites, electronic journals, scientific databases, corrosion education and mailing lists..  Of the corrosion information sites, Corrosion Doctors and Corrosion Source have been found to be the most useful.  The www.jcse.org site is a site to which scientific papers can be submitted for live refereeing. 

Web sites of organisations such as NACE and CDA (www.copper.org) contain information on corrosion, while www.csa.com and www.cas.org run scientific abstract services, although the latter two require a subscription payment.  On-line courses for corrosion engineering qualifications are run in the UK by www.umist.ac.uk which can lead to an MSc in Corrosion Control Engineering.                      

General Web Pages

•      Corrosion Doctorswww.corrosion-doctors.org ;  Written by Pierre Roberge “More than 1000 pages”10000 visitors per month

•      CorrosionSource  -  www.corrosionsource.com ;   Written by InterCorr;  Part free, part commercial/restricted access; “Over 30000 pages”;  Pages can be a bit outdated

•      Corrosion Information Serverwww.umist.ac.uk/corrosion/cis ;   Set up by Stuart Lyon; Largely a collection of links (some out-of-date); Useful (and up-to-date) Corrosion Diary

•      Corrosion.comwww.corrosion.com;  Largely concerned with coatings;  Fairly active coatings and corrosion forums

•      NACEwww.nace.org;  Mostly NACE activities;  Navigation can be difficult;   Archive of NACE Corrosion Network

•      ICorr -   www.icorr.demon.co.uk;   Site requires the Java Virtual Machine to be installed – if not it seems to be seriously broken (go to www.java.com to install JVM)

Electronic Journals

•      Open/free -   Journal of Corrosion Science and Engineering (www.jcse.org)

•      Commercial/paid for -   Corrosion Science (www.elsevier.com);   Corrosion (www.nace.org);   Corrosion Engineering Science and Technology (formerly British Corrosion Journal) (www.maney.co.uk);   Corrosion Management (www.icorr.demon.co.uk)

Databases

•      Cor-Sur (www.knovel.com)

•      Copper & Copper Alloy Corrosion Resistance Database (www.copper.org)

•      Corrosion Abstracts (www.csa.com)

•      Metadex (www.csa.com)

•      SciFinder (online version of Chemical Abstracts www.cas.org/SCIFINDER/scicover2.html)         

Courses

•      Corrosion Doctors

•      UMIST Online CoursesIntroduction to Corrosion (12 weeks);  MSc in Corrosion Control Engineering (nine 12 week modules + research project)www.ut.ac.uk/corrosion

Mailing Lists and News

•      NACE Corrosion Network -  Subscription information and access to archives on www.nace.org/nace/content/discussion/ncnindex.asp ;   Very active (about 180 messages in January 2004);  Largely corrosion engineering focus;   Also Coatings Network

•      CORROS-L   -  Subscription information on http://www2.umist.ac.uk/corrosion/corros-l/ Tends to be a bit more academic than NCN;  Lower throughput (22 messages in January 2004)

Standardization Activities

•      OCEAN – Open Corrosion Expertise Access Network  - EU Funded Thematic Network;  Devised metadata standards for marking web pages that contain corrosion information

•      MatMLXML Schema for describing materials information;   NACE may work on corrosion addition

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3.1  ‘Control of Cathodic Protection on CRA Pipelines’, Robin Jacob (The Corrosion Consultancy Limited) 

Conventionally designed sacrificial anode systems produce potentials more negative than those strictly necessary to protect steels from seawater corrosion.  On pipelines, which are invariably coated, potentials will be close to anode potential [ca. -1080 mV1].  Such negative potentials can have a number of undesirable effects, including hydrogen embrittlement of some engineering materials  and premature deterioration of coatings.  As some pipelines are now being constructed from corrosion resistant alloys, needed to handle corrosive internal conditions, consideration needs to be given to the possibility of hydrogen embrittlement. 

The presentation describes a method of controlling the potentials produced by sacrificial anodes to values closer to those strictly required for protection.  By the use of Schottky barrier rectifiers [SBR] in conjunction with the anodes, together with careful design, potentials in the range -775 to -825 mV can be ensured.    At these potentials, the risk of embrittlement is much reduced. 

In order to establish a design methodology for such limited potential systems, a trial has recently been carried out using different types of SBR in a number of configurations. Sufficient information is now available to allow the design of limited potential cathodic protection systems operating within a closely controlled potential range.  Data from a CRA pipeline fitted with potential control will be presented. 

[1 All potentials stated relative to the Ag|AgCl|seawater reference electrode]

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3.2   Keynote lecture:  ‘Three New DNV Recommended Practices on Submarine Pipeline Corrosion Control’, Tomas Sydberger (DNV, Hoevik, Norway) 

The following documents were addressed: 

  1. DNV RP-F102 (2003): “Pipeline Field Joint Coating and Field Repair of Pipeline Coating”
  2. DNV RP-F103 (2003): “Cathodic Protection of Submarine Pipelines by Galvanic Anodes”
  3. DNV RP-F106 (2003): “Factory Applied External Pipeline Coatings for Corrosion Control”

The presentation briefly described the contents and philosophy of the three documents and how they relate to other DNV documents on submarine pipelines, as well as other national and international codes and standards.  Emphasis was made on DNV RP-F103 for which cathodic protection (CP) design calculations have been performed for an oil export pipeline with asphalt enamel + concrete coating and, furthermore, a production flowline with a thermally insulating coating based on fusion bonded epoxy and polypropylene.  The results were compared with those using other CP design codes, including DNV RP B401, NORSOK M-503 (1997) and ISO/FDIS 15589-2 (2004).

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