TRADITIONAL SHIP COATINGS STRUGGLE TO PREVENT CORROSION AFTER HYDROJET PREPARATION

Left: High-pressure hyrdojet preparation of a bulk carrier’s hold. Right: Application of Steelpaint’s Stelpant after hydrojetting

German steel protection specialist Steelpaint says the growing use of hydrojetting across the global ship repair sector is increasing demand for coating systems that can perform on prepared surfaces that are difficult for conventional two-component paints.

Hydrojetting, also known as water jetting or water blasting, uses ultra-high-pressure water at 700–1,000 bar to clean, descale and strip coatings from hulls, decks, cargo holds and tanks.

It is the main alternative to dry grit blasting during drydock maintenance, which is under increasing scrutiny due to its impact on local marine environments.

Unlike abrasive blasting, hydrojetting eliminates dust and spent abrasive while reducing contaminated waste into the surrounding environment.

However, Steelpaint says it also creates challenges for conventional coating systems. Because hydrojetting does not create a new anchor profile on steel, residual moisture and flash rust can affect adhesion and curing, according to Steelpaint technical director Andreas Engert.

He says traditional coatings can struggle to perform reliably under these conditions, especially in hot and humid climates where relative humidity often exceeds recommended limits.

“If conditions are not ideal, such coatings can fail to adhere properly, blister, crack or break down,” he says. “Single-component moisture-cure polyurethane coatings like our Stelpant and Stelcatec systems can overcome these limitations.”

Engert says this advanced type of coating system can be applied at relative humidity levels of up to 98%, without dew point restrictions, and are suitable for damp surfaces and areas with light to medium flash rust. They are also designed to maintain strong adhesion on partially corroded or relatively smooth steel surfaces.

“These single-component coatings are well suited to onboard maintenance, where conventional surface-preparation standards are often impractical”, he says. “These coatings require no mixing and can be applied by brush, roller or spray to damp or manually prepared steel.”

Designed for temperatures between -5°C and 50°C and relative humidity up to 98%, they adhere to all kinds of steel including stainless, galvanised and aluminium, allowing corrosion protection work during drydocking or while vessels remain in service.

“Single-component moisture-cure polyurethane coatings can be applied to hydrojetted steel prepared to SSPC-SP WJ/NACE WJ (WJ-3 – WJ-2) standards, as well as vapour-blasted and manually or power-tool prepared surfaces”, Engert says, adding that steel coated with a moisture-cured system and prepared only by high-pressure hydrojetting can deliver “better long-term performance than conventional materials applied to more extensively prepared substrates”.

Hydrojetting has gained particular acceptance where waste disposal, environmental controls or operational constraints make abrasive blasting less practical.

Steelpaint points to government-owned shipyards in China, in particular, that have widely adopted hydrojetting as their preferred means of steel surface preparation.

Steelpaint director Frank Müller said: “By combining hydrojetting with moisture-cure coating technology shipowners and offshore operators benefit from an alternative corrosion-protection method that can simplify maintenance, reduce maintenance costs and extend service life in demanding conditions.

“Sustainability goals are optimally supported, as waste is reduced and resources are saved during surface preparation, additionally the single-component properties of the coating material also reduce waste due to less packaging.”

Advantages are also evident in terms of working hours, as the mixing process of the usual two-component products is eliminated, and since the products have no pot life, waste generated by mixed but unused two-component products is further reduced.

Steelpaint’s moisture-cure coating technology has been in commercial use since 1982.

■ Contact: Steelpaint GmbH www.steelpaint.de/en/