Interesting Findings on the Osmotic Blistering of Paint on Ship Hulls

A PhD thesis at the University of the Basque Country (UPV/EHU) focused on the study of blistering of paint on large submerged structures such as ships. The author of the thesis is Mr Raúl García Bercedo and his work entitled, Study of osmotic blistering in marine paints applied to non-metallic substrates, in conditions of total immersion.

This type of fault with the system of painting, i.e. blistering, is very frequent and due to the presence of soluble salts between the ship's hull and the paint. The most common salts are sulphates and chlorides. Sulphates occur in industrially contaminated environments and chlorides are typical of marine environments. The latter were the subject of this research, as these salts adhere to the hull of the ship, arising from the saline environment of the shipyard slips where the painting operations take place. These chlorides, after painting, remain trapped between the steel hull and the coating of paint. When the vessel puts to sea, it remains submerged in water with a salinity that varies depending on the zone of navigation. The deterioration of the paint is very rapid when the critical threshold of saline contamination is passed, and the osmotic blistering appears.

Osmotic blistering

Mr García analysed osmotic blistering and the influence of passing time and the saline concentration of the water in which the paint is submerged, as well as the concentration of salt between the ship's hull and the coating of paint and the thickness of paint. He also determined the critical values of concentration of chlorides for the occurrence of this blistering.

According to Mr García, the greatest achievement of this research was the calculation of limit values for the saline concentration for avoiding paint deterioration and thus impeding the loss of anticorrosive protection of the ship's hull. The critical values for contamination of chlorides in order to produce osmotic blistering is greater than 10 mg/m2 and less than 25 mg/m2. Moreover, Mr García obtained data for the influence of the thickness of the paint and the time of immersion in water, the adherence of paint to the hull and the onset of this type of fault. Amongst other things, he concluded that the greater the thickness of the film of paint, the more the onset of osmotic blistering is retarded. Increasing the thickness of the film of paint reduces its permeability. In any case, once the blistering process has started, it was observed that it develops independently of the paint thickness. As regards the time of immersion, he concluded that, it is the first four months when the most important changes in the blistering take place. Finally, Mr García observed that the beginning of the osmotic blistering process does not depend on the adherence to the substrate. Once initiated the process of blistering, the less the adherence of the coating of paint, the greater the degree of blistering.