Leaking Valve Investigation

​ESR Technology routinely performs failure investigations on static plant and equipment. This case study investigation into a leaking metering valve at a chemical plant is typical of the work undertaken.

The chemically aggressive nature of the carrier fluid, damage to the paint layer and jetting of chemical leakages onto the surface were all considered as possible causes of the corrosion damage.

Firstly a visual inspection was carried out. The corroded area on the external surface was relatively large, but a rectangular shaped area within this had a concentrated distribution of deep corrosion pits. Within two of the larger corrosion pits, full wall thickness pin holes were identified. Raised regions were identified at either end of the rectangular shaped pitted area. Removing the dome-end cap revealed the internal surface of the valve remained in excellent condition with no evidence of corrosive attack.

Optical microscopy of metallographic sections through the pitted area revealed material loss on the outer surface with no degradation on the inner surface. The raised areas at the end of the rectangle were found to be remnants of non-penetrating tack welds. The welds and the concentrated rectangular area of corrosion pointed to the presence of an identification label which would have originally been attached to the valve by the manufacturer or end user.  Further tests confirmed that the valve was manufactured from low carbon steel.

The attachment of such labels can lead to the initiation of corrosive attack through the creation of crevices. Electrolytes collect and stagnate in crevices and the restriction of oxygen creates a differential aeration cell. The electrolyte then becomes extremely aggressive and locally corrodes the metallic surface. In cases where different metals are galvanically connected, the less noble material preferentially corrodes. Corrosion and hence material loss occurs within the crevice through pitting attack which can be a rapid process. Furthermore, in specific environments pits can also acts as stress raisers for other mechanisms such as stress corrosion cracking (SCC).

The attachment of a label in this case resulted in the formation of a crevice and hence pitting corrosion beneath. Typically an identification label would manufactured from pressed stainless steel; a more noble material than low carbon steel. Full wall thickness pits then resulted in the detected leakage. Care should be taken to assess galvanic compatibility and whether sealants should be used to remove crevices when attaching identification labels.

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