Metal Spraying – What is it exactly?

Zinc metal spraying is a highly effective corrosion prevention technology widely used on naval vessels, offshore equipment and civil structures such as bridges. The process can be thought of as being similar to galvanising as it results in a protective layer of zinc being bonded to a prepared steel substrate. Somewhat confusingly, metal spraying is known as a “cold process” despite the fact it involves the application of molten zinc. This is to distinguish it from galvanising, which requires the dipping of steel components in a bath of molten zinc at temperatures of 450C with the associated risk of heat distortion. Aside from the impracticability of dipping an entire narrowboat hull into a 450C bath, metal spraying provides similar protection without this risk of heat distortion.

How is all this relevant to the narrowboat owner? Well, we’re used to having to fit sacrificial zinc or magnesium anodes to help protect a narrowboat’s hull from pitting, but we also know how localised this protection can be. Metal spraying effectively turns the entire hull into an anode. This means that when the blacking starts to wear or is damaged by underwater obstacles, the steel is still protected and the risks of pitting are substantially reduced.

The Process In Detail

The first stage in the process is to lift the boat and pressure wash the hull to remove accumulated wildlife like weed, mussels and general dirt. This is normally done on a Monday as the whole process typically takes a week to complete and means we can lift back in on the following Monday having given the freshly blacked hull the weekend to harden off.

Next, the boat is moved to our purpose designed grit-blasting and metalisation facility. Steel lined with underfloor heating, state of the art dust extraction and all-round lighting, this new facility was installed in 2013 and to the best of our knowledge is the only one of its type in the country.

External fittings such as anodes and bow-thruster grilles are removed and a protective shroud is lowered over the cabin-sides and securely taped to the gunnels to protect the topsides from the grit-blasting process. As a further protection measure, low pressure air is fed into the shroud to maintain a positive pressure against any grit ingress.

Grit Blasting

Both baseplate and hull sides are grit-blasted using a bone-dry, chilled iron grit. (It’s important to note that we never use sand or other commonly used materials that might impart moisture to the freshly blasted hull). The grit-blasting process removes all traces of rust and/or millscale leaving the surface ready for the molten zinc spraying process.

Before starting the metal spraying process, the hull is closely inspected for any signs of damage or significant pitting of the surface. Spot welding pits, for example, is much better carried out at this stage as the grit-blasted metal provides the best possible surface for the welding process.

Metal Spraying

The final stage is the metal spraying itself. Pure zinc wire is vaporised at very high temperature to form a molten metal plasma which is directed at the grit-blasted steel. Entering every microscopic pore in the surface of the steel, the molten zinc binds tightly, until the whole hull is covered. The thickness is checked with a dry-film meter to ensure a minimum of 100 microns is achieved across the job.

So, there you have it, that’s molten zinc spraying explained. Why would you want to invest in it? Well, if your boat is new or nearly new it’s probably the single most effective investment you can make in ensuring your hull enjoys a lifetime free of pitting. If your boat is older and showing its age, a little bit of remedial work such as spot-welding pits, coupled with zinc metal spraying will significantly prolong its useful life and stave off the day when re-plating might be the only remaining option.

Whilst metal spraying alone will protect your hull from corrosion, most people don’t want to cruise around with a silver coloured hull, so after completing the process hulls are normally blacked. At Debdale Wharf we offer a choice of regular bitumastic products or 2 pack epoxy coatings, providing the ultimate in hull protection. See our Blacking page for details. A significant advantage of a metal sprayed surface is that it acts like a sponge, soaking up top coats and creating a tremendous bond between the metal coating and paint or blacking.