A Recording Instrument For Measuring Corrosivity In Offshore Seawater

ABSTRACT An instrumented test buoy has been developed for simultaneous recording of hydrographic data with corrosion data. The instrument will give information on instantaneous corrosion rate and cathodic protection requirements as a function of-ocean current and time. The instrument has been tested in arctic offshore water W of Greenland at depths of 30-300 metres, where high corrosion rates and CP requirements were found. With recordings every 10 minutes, a close correlation between corrosion rate and tidal movements was found. INTRODUCTION The corrosion rate of steel in sea water in various geographical localities is usually determined from the weight loss of a steel panel, which has been exposed for a given length of time, often one year or longer. The results of such tests do only partly reflect the "Corrosivity" of sea water, and they do not provide much of the information needed by the designer of marine structures. To get a clearer picture, one might say, that the corrosion of steel (or other metals) in a given marine application is dependent on the following five "environmental" factors:The chemical environment ("corrosivity" of sea water): Oxygen content of the water pH (mainly because if reflects the chemical balance of type hardness forming components of the water), Chloride concentration, Temperature, (+ other less important factors)The physical environment: Water velocity, Erosion by particles, air bubbles and ice, Splash zone conditions (aeration, drying out).The biological environment: Biological fouling, which includes bacterial slime, algae and animal fouling and the combined effect of this fouling on the chemical and physical environment at the steel surface.The engineering environment: The formation of corrosion cells on structures extending through zones of water representing different environments as defined above. The formation of galvanic cells in structures with different metals, including the effect of welds, covering with concrete, cathodic protection etc. Local effect of crevices and similar geometric factors. Mechanical effects (fatique, stress corrosion).Time: The corrosion rate will diminish with time, due to the formation of a layer of corrosion products and fouling, which will act as a barrier to the diffusion of oxygen. A closer analysis will show, that the influence of all these factors on the corrosion of steel can be interpreted as their effect on the access of oxygen to the steel surface. The corrosion of steel is determined by the amount of oxygen, which is consumed in the cathodic reaction, and for normal unalloyed steel this corresponds to all the oxygen reaching the steel surface. This is true both for small test panels and for large structures, but corrosion cells will cause a redistribution of the corrosion on most structures, often resulting in local corrosion on areas with limited access of oxygen.