Casestudy: Pitting corrosion on an aluminum hull
The client asked the help of Endures BV on a corrosion problem for one of their aluminum catamaran. The vessel had been purchased about 1 year prior to the case but had been used by previous owners for about 15 years. Some leakages were noticed by the client and during inspection, some holes have been seen on the aluminum hull of the vessel.
Our client delivered several damaged parts and sent it to Endures. When analyzed in our laboratory, a multitude of pits were observed in- and outside of the hull. A full corrosion failure analysis (analysis of corrosion products and elements present on the samples, preparation of the material, cross sections, SEM-EDX, microscopy), was applied to identify the cause of the damages and the type of corrosion involved. Other possibilities for different corrosion mechanisms were investigated.
Figure 1: Aluminium plate with 2 perforating pits, circled in red.
Figure 2: 3D picture of a perforating pit, with details of the pitting steps
Multiple pits were found throughout all samples brought by the client. Those pits were present mainly on the inside of the hull and the biggest were completely going through the whole material (Figure 1), with all the steps visible on 3D pictures (Figure 2). Contaminants such as chloride and Sulphur were present which can be found in seawater but also in cleaning chemicals, when cleaning the inside hull of the vessel. One of the protective anode was also not made of the right material. Based on this data and on the information given by the client, the cause of the damages was attributed to pitting corrosion. Endures advised the client to visually inspect the inner side of the hull especially at the contaminated compartments and to clean to observe the location of the pits. The most attacked plates need to be replaced. Compartments where only a few pits are found, those should be repaired by drilling a hole until the bare substrate is visible and welded. Slight pitting on the outside of the hull can be grinded and after correct surface preparation applied with the prescribe coatings.
Casestudy: Galvanic corrosion and mass loss of watercooler pipes on an offshore patrol vessel
On several patrol vessels, the hot zinc dipped steel pipes right next to the stainless/titanium heat exchangers were perforated. The pipes are attached to the heat exchangers using different materials: Steel, Stainless Steel and Titanium. Inspection of the inside of the pipes showed heavy corrosion and indicated that similar perforations could occur. Pipe samples analyzed in the laboratory showed a thick layer of corrosion products throughout the pipe (Figure 1 and 2).
A full failure analysis package, including preparation of samples for microscopy, cross cut examination, SEM-EDX and microbial analysis (MPN technique), was carried out to identify the root cause of damage and type of corrosion involved. The pipe was cut into four sections with section 1 starting at the pipe/flange connection. Analysis showed a thick layer of corrosion products had formed throughout the interior of the pipe adjacent to the flange. As seen in Figure 2 internal pipe corrosion progressively diminishes as you move away from the flange.
Figure 1: Heavy corrosion inside the steel pipe connected to the stainless/titanium heat exchanger.
Figure 2: Reconstruction of a half steel pipe, with details of zinc layer presence
Micro-organisms were found to be present but not active, and are therefore not involved in the corrosion mechanism. Corrosion products found on the inside of pipes are common for this type of steel. After removal of the corrosion layer the protective zinc layer that should be present was found to be missing on part of the pipe closest to the heat exchanger. On this part significant mass loss was observed. Furthermore corrosion pits were found not only close to the heat exchanger but throughout the whole pipe. From these data and pictures from other parts of pipe systems on board this ship, in conjunction with review of materials used in this system it was concluded that the damage can be attributed to galvanic corrosion. Based on this advice was given for proper isolation of the steel pipes from the stainless/titanium heat exchanger.