History

A nest of 10 rectangular GRP tanks for storing hydrochloric acid gave trouble free service for 5 years until one failed and released 1000 litres. This occurred in an enclosed factory space with poor ventilation. The acid fumes condensed on structural, process, and electronic/electrical metalwork, initiating rapid corrosion and the need for a very expensive, extensive, and intensive clean up operation.

Inspection and analysis

The failed tank is shown in Figure 5.17. A sidewall section of the tank was removed and a polished cross-section inspected. The wall was 12 mm thick including a corrosion barrier of approximately 1 mm thickness. The removed section was submitted to an ashing test. The residue was chopped strand glass mat (CSM) with an inner veil of C glass. Weight loss via ashing indicated that the tank laminate had a glass fibre content of 30% by weight.

The wall thickness and laminate construction, and the size and location of sidewall stiffening struts satisfied all the requirements of BS 4994 (Specification for Design and Construction of Vessels and Tanks in Reinforced Plastics).

The fracture surface was inspected. This was predominantly rough with extensive fibre pull-out. A smooth, semi-elliptical area 100 mm in length at the inner surface and 20 mm in length at the outer surface was identified as the fracture initiation region.

The ashing test was repeated on samples taken from the cracked corner. The C glass veil was found to be absent.

Figure 5.17 The failed GRP tank

Failure diagnosis

The failure is due to acid induced stress corrosion cracking. The C glass veil has two purposes. Firstly it acts to arrest crack growth through the laminate, thus protecting the structural E glass against acid exposure. Secondly it reduces shrinkage strains resulting from resin cure. Unreinforced polyester resin will shrink in volume by at least 4% on curing. Homogeneous volume shrinkage is prevented by the constraint of the male mould and the result is a residual biaxial tensile strain. In this case it is likely that the residual strain plus the applied strain (due to hydrostatic pressure of the contents) exceeded the critical strain for resin cracking (about 2%). The structural reinforcement then suffered rapid stress corrosion via direct contact with the acid.

Lessons and consequences

1. Conventional GRP structures used for acid containment rely upon the protection given by an effective resin rich corrosion barrier.
2. Polyester resins described as rigid or resilient need to be at least lightly reinforced with C glass or synthetic fibre to resist crack initiation and growth. It is essential that the reinforcement is continuous and particular care is needed to ensure that potential discontinuities do not coincide with high applied stress. For processes that cannot incorporate reinforcement in the corrosion barrier (e.g. centrifugal casting), high strain at break 'flexible' resins must be selected.
3. An internal inspection of the failed tank and the other 9 tanks revealed all other corners to be free from surface cracking. Subject to detailed annual inspection the remaining tanks were deemed to be fit for service.