IIMS has published a new free 36-page informative guide called ‘The use of moisture meters on small craft’. Additionally, the Institute has republished another free 16-page guide entitled ‘Biological Attack on Iron & Steel’. Both guides, authored by Jeffrey Casciani-Wood HonFIIMS, are available in pdf format from the IIMS website.
The use of moisture meters on small craft
The first thing to know about moisture meters is that they do not actually measure moisture. What they do measure is conductivity. The origins of the moisture meter lie in the building and construction industries and the original scale was based on the water content of brick and stonework. The scale has largely remained unchanged. There are a number of these machines available in the market and they were first introduced into the marine industry for checking how an frp hull had dried over time prior to rebuilding for osmosis treatment and for that they remain a useful tool. They are also used to check for moisture below a surface that looks dry.
Electrical moisture meters have an enormous advantage for the general survey as they are clean and nondestructive, but they do have limitations and they do NOT quantitatively measure moisture on/in hull substrates. The majority record electrical resistance between two applied electrodes or capacitance and, more recently, some measure the reflection of radio frequency emissions from the meter.
Biological Attack on Iron & Steel
Boat owners and marine surveyors will be familiar with common iron rust whatever form is takes. The literature on the subject of electrochemical or galvanic corrosion is enormous. However, although the phenomenon is well known to the mining and oil industries where it causes millions of dollars worth of damage annually, biological attack is not so widely understood in the marine world.
Macrobiological attack is the phenomenon of mussels, barnacles, slimes, grasses and seaweeds attaching to a vessel hull. These items do not usually cause serious harm to the metal but they can and do slow the boat down and increase the fuel consumption. However, there is a different kind of corrosion which is also found on boat hulls, particularly those lying in water such as canals or rivers containing decaying vegetable matter. Few people are aware of the problem or realise it is caused by microbiological attack, or metal worm.
Metal worm is a highly unpredictable process but the marine surveyor should realise that, under the influence of microorganisms, corrosion processes can happen in a matter of months compared to the years it would take for ordinary abiotic corrosion to reach serious proportions.