After the drink she needs soda

The Navy's first submarine will be submerged again to wash off the sea's corrosion,

"From the point of view of a museum conservator, it's bleeding to death," says Chris O'Shea of the blood-red stains dripping down the sides of the Holland I. In fact, the stains are not blood but ferric chloride, a sign that the Holland I is rotting away.

The Holland I was the Royal Navy's first submarine. Compared to today's models, she was extremely basic - a bucket for a toilet, a maximum depth of 60ft and an eight-man crew that lived in extremely cramped conditions. But, according to Commander JJ Tall, director of the Royal Navy Submarine Museum near Portsmouth, where she resides, the Holland I is "the mother of all submarines. The design of the Holland determined the shape of all submarines today, including the mighty Tridents".

Built during the British-German arms race of the early 20th century, the Holland sank on the way to the scrap yard after 10 years' service. She then spent some 70 years corroding at the bottom of the sea before being raised in 1980.

Unfortunately, her initial washdown and lick of anti-corrosion paint failed to stop the rot. To make matters worse, the paint has entombed the chlorides from sea salt (sodium chloride) which corrode the iron body. And it doesn't stop there: the coating also allows water vapour and oxygen in the surrounding air to pass through to the metal and accelerate the process.

"You can compare the ferric chloride to cancer cells," says Mr O'Shea. "As long as the chloride is present, this corrosion will continue rapidly."

The answer is to immerse the whole submarine in a huge tank full of a solution that will rid it of all damaging chlorides. Many chemical solutions will do the trick but the museum chose sodium carbonate - better known as washing soda.

"It's environmentally friendly and can be pumped into the drains when we've finished with it," Mr O'Shea explains. "The water and sodium carbonate solution will be pumped around the tank for two, three, maybe even four years - we don't really know yet. We'll monitor the solution on perhaps a monthly basis. The chloride level will determine the overall period of washing. We haven't got a clue exactly how long it will be, but looking at the amount of corrosion going on, there's an awful lot of salt to come out."

But before the washing can begin, that shroud of paint must be removed. To do this, a new process is being used. It is similar to sand-blasting - firing grains of sand at something to scour its surface. Only the new process uses bits of sponge-like material impregnated with a mild abrasive.

"It wipes off the paint like magic, it doesn't create any of the dust you usually get with grit blasting and there's no health hazard. What's more, it's recyclable. We just hoover the material up and put it back into the machine."

The main danger to the Holland at the moment is the corrosion of the rivets that hold it together. If they were to vanish, the craft's iron plating - no matter how healthy - would fall off. Certain rivets look metallic but there is no iron in them at all, just various oxides of iron. There are even globules of hydrochloric acid where the chlorides from the sea salt have reacted with the hydrogen in water.

Despite the Third Sea Lord of 1900 declaring submarines "underhand, underwater and damned un-English", Britain soon had one of the best fleets in the world. It seems ironic that, with the Holland I at least, the Royal Navy's battle is no longer with any foreign enemy but with the ravages of time and tide.