Case study 1: Maritime infrastructure
Port authority in the North of the Netherlands
Investigation of a MIC failure case in a marine/brackish harbor in The Netherlands. The quay wall consisted of an old part without and a new part with sacrificial anodes. Aim of the investigation was to find the root cause of failure of sheet pilings in the old part.
To this end a comprehensive failure analysis package was carried out on the structure:
- coupon investigation on corrosion damage,
- microbial analysis for corrosive organisms,
- chemical analysis of corrosion products,
- potential measurements and
- Wall thickness measurements.
Such approach will allow a thorough diagnosis and analysis for concluding if MIC is really the root cause of the damage observed. From this also proper recommendations can be given for mitigation of the problem.
After inspection of the quay wall potential and wall thickness measurements were performed. Next to it corrosion product samples were taken for microbial analysis and microbial related failure analysis (SEM-EDX and volatile sulfide determination).
Results showed that corrosion related microorganisms were frequently found in corrosion products scratched from the surface. Sulfur was found in corrosion products indicating possible activity of SRB directly at the damaged surface and detection of gaseous sulfide from corrosion product samples confirmed this. Potential measurements showed variable values between -0.7 and -1.02 V vs sat Ag/AgCl. Sacrificial anodes were placed only on the new part of the structure (D1).
Along the older part the potential was not low enough, resulting in severe corrosion damage and large perforations whereas the new part was adequately protected by the sacrificial anodes.
All this leads to the conclusion that MIC plays a role in the observed damage of the quay wall structure and that the part where anodes are installed are protected against microbial activity if the potential is lower -0.9 V vs sat Ag/AgCl .
The old structure should be cleaned and sacrificial anodes should be placed as well on the old parts of the quay wall until it will be renewed in future. Potential should be monitored and a value lower than -0.9 V vs sat Ag/AgCl should be present to protect the structure at the inspected location under the given environmental circumstances.
Case study 2: Fast corrosion and unexpected mass loss of quay walls in a harbor structure
Harbor and infrastructure administrator, engineering bureau
A visual inspection of a quay wall indicated that microbial-induced corrosion was involved in pitting damages. Adjacent to the holes, corrosion products were found consisting of brown-blackish layers which could easily be removed from the steel surface. The steel was shiny underneath. A combined approach of microbial analysis (MPN, q-PCR) and corrosion failure analysis (cross sections, SEM-EDX) was used to identify and quantify any microorganisms present as well as their activity causing MIC damages. Additionally, the possibility for other corrosion mechanisms was investigated.
Due to the application of different techniques (potential measurements, cross sections, microscopy) corrosion mechanisms such as stray current, could be excluded for the specific structure. However, presence of corrosion-relevant microorganisms was confirmed. Based on this data combined with the results gained from the failure analysis, MIC was detected as the corrosion mechanism that resulted in fast localized corrosion. The end result was holes several diameters wide scattered over the structure.