Preferential weld line corrosion (also known as grooving corrosion, knife-line attack or trench-like corrosion) is the selective and rapid corrosion of a weld or bond line. The corroded area formed is groove shaped, and is thus a potentially severe defect. It tends to form a relatively sharp notch in material which is also usually less tough than the parent material.
Kato et al [1] suggested a mechanism for grooving corrosion in electric resistance welded steel pipe, as follows. Rolled strip typically has a banded structure, which is upset during the electric resistance welding process. This means that the laminar structure of the steel is exposed to the pipe surfaces following welding and bead removal, as are any rolled-in inclusions contained in the laminar structure. It is thought that grooving corrosion initiates in the steel matrix surrounding inclusions (mainly MnS), and further corrosion occurs by removal or dissolution of the inclusion itself, thus revealing further inclusions available for selective corrosion, and hence a groove forms. The areas around the inclusions (at which the corrosion initiated) were found to be rich in sulphur. The presence of this sulphur was suggested by Nejihashi et al [2] to develop at the weld by rapid reprecipitation of MnS after dissolution during welding. The weld area was said to be anodic in comparison to the base material hence forming a macro-cell between the narrow weld and the base metal leading to grooving corrosion.
The principal methods of controlling grooving corrosion are the adjustment of the steel composition and heat treatment and indeed very low sulphur levels are necessary to ensure good performance in water service. Since the S-enriched regions around MnS inclusions appear to be the initiation sites for grooving corrosion, it has been suggested that a reduction of the S content helps to prevent initiation [2]. It is also considered to be important to modify the S inclusions and that the addition of Ca leads to the formation of CaS inclusion which are more chemically or electrochemically stable than other sulphides.
Other suggested modifications to the steel compositions are the addition of Cu and/or Ni. Nejihashi et al [2] suggested that elemental additions of Cu and Ni cause capturing of S ions (released due to the high temperature of welding) on the corrosion pit surface, resulting in the formation of a non-soluble film of Cu2S and Ni2S3 in the rust layer of the steel, thus suppressing corrosion. However, there might be some concerns over the efficacy of such techniques in abrasive service conditions.
Seam weld heat treatment is generally found to reduce or prevent grooving corrosion. Nejihashi et al [2] suggest that the application of heat treatment could reduce the S-enriched zone around the MnS inclusion, lessen the structural differences between the weld and the matrix, reduce the residual stresses at the weld, and reduce the macrocell formation.
References
| 1. |
Kato C, Otoguro Y, Kado S, Hisamatsu Y, 'Grooving corrosion in electric resistance welded steel pipe in sea water', Corrosion Science, 1978, vol. 18, pp 61-74. |
| 2. |
Nejihashi S, Nishi H, Tomizawa Y, Tamaki K, Watanabe S, 'High Frequency Electric Resistance Welded Pipe for Offshore Application', Kawasaki Steel Technical Report No.19, Nov 1988 |
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