Metals

Internal Corrosion of Stainless Steel Water Pipes Near the Weld Bead

Customer: ABB inc.

Mandate submitted to CIR Laboratory:

Solving the problem of corrosion observed in service near the weld bead on the inside of stainless steel water pipes.

Work conducted by CIR Laboratory:

1) Sampling and macroscopic observation

Samples were taken from the corroded pipes in order to examine the appearance of their inner surface using a binocular microscope. It was thus possible to observe the presence of several corrosion pits in the vicinity of the weld bead in the heat affected zones, which result from thermal cycling imposed on the base metal during the welding operation.

2) Microscopic samples observation and elemental analysis

The samples were then observed in the heat affected zones by means of a scanning electron microscope (SEM) equipped with an elemental analysis system. Based on the analysis results and with reference to the literature, it could have been concluded that the secondary phase observed in the grain boundaries in this zone was chromium carbide.

3) Corrosion mechanism identification

The internal corrosion of the water pipes in service has been associated to the absence of a protective passive layer near the weld bead in the heat affected zones. The absence of passivation was caused by the precipitation of chromium carbide at the grain boundaries in this zone, thus making them anodic and therefore responsible for the corrosion problem observed in service.

4) Solution to the corrosion problem

The proposed solution to the corrosion problem of the water pipes was to submit them, after welding, to an homogenization heat treatment followed by quenching under conditions promoting their passivation.

Livrables:

The homogenization temperature as well as the quenching conditions requiered were reported to the customer. The name of a supplier where the required heat treatments can be made and thus allowing the customer to recover its existing water pipes has also been reported.