THE WEINS NUMBER--INTEGRATED LONG TERM CORROSION DECAY OF IRON BASED ALLOY SHIPWRECKS AND ARTIFACTS ON THE SEAFLOOR

Knowledge of the condition of submerged vessels  is important as related to  the possible contamination of the environment from fuel oil discharge and to marine wrecks or artifacts  under study for preservation. Based on comparisons  of corrosion and environmental data from shallow  and deep water sites, the  Weins Number concept is proposed  to predict long term corrosion of iron based alloys in sea water.                                                                                                                                                                                                                       Corrosion studies, as a component of the National Park Service Submerged Resources Center’s USS Arizona Preservation project, led to pioneering development of a minimum impact methodology to determine the corrosion rate of steel shipwrecks and artifacts in seawater.  The methodology, Concretion Equivalent Corrosion Rate (CECR),  involves the physical and chemical analysis of overlying concretion (hardened marine biofouling) and conversion of the data to corrosion rate,  i_corr , of substrate metal. Using the results from USS Arizona at Pearl Harbor, the technique was applied to a Japanese Midget Submarine, tentatively identified as I-20A, located in 400 m of water 2 km offshore from Ohau. The results showed a factor of five decrease in the actual corrosion rate from  warmer shallow water in Pearl Harbor to the colder deep water offshore. Apparent corrosion rates, calculated from diffusion of oxygen through the concretion at the same sites, showed the opposite, a factor of nearly four increase from warmer shallow to colder deep water. Based on these comparisons, the ratio of the actual corrosion rate, i_corr, to the apparent oxygen corrosion rate, i_aocr , was extended to two additional sites, Sub Marine Explorer, located in the tidal zone, St Elmo’s Island, Bay of Panama and  a recently visited  5^th Japanese Midget Submarine, I-16A,  also located offshore from Ohau.  Applying the Arrhenius law, the corrosion rate ratio i_corr/ i_aoc was found to be nearly  linear as a function of reciprocal absolute temperature.  Based on these correlations,  the dimensionless ratio is defined in terms of the Weins number,Wn,  similar in concept to engineering dimensionless ratios such as the Reynolds number,Re,applied in fluid dynamics.

From measurement of temperature, salinity, oxygen concentration and concretion thickness in situ and determination of Wn from the Arrhenius plot, the corrosion rate can be determined. Although the data for the  plot is limited, the Weins number concept is being tested, verified and refined  by developing a broader base of compatible measurements.

The USS Arizona Preservation Project is funded by the Department of Defense, Legacy Resource Management Fund, National Park Service Submerged Resources Center, USS Arizona Memorial, Hawaiian Undersea Research Laboratory, University of Hawaii, National Oceanic and Atmospheric Administration and the University of Nebraska Foundation. The Weins number is named in memory of Dr.William N.Weins, Department of Mechanical Engineering, University of Nebraska-Lincoln.