The changes of corrosion potential (E corr ) of metals immersed in seawater were investigated with electrochemical technology and epifluoresence microscopy. In natural seawater, changes of E corr were determined by the surface corrosion state of the metal. E corr of passive metals exposed to natural seawater shifted to noble direction for about 150 mV in one day and it didn't change in sterile seawater. The in-situ observation showed that biofilms settled on the surfaces of passive metals when E corr moved in noble direction. The bacteria number increased on the metal surface according to exponential law and it was in the same way with the ennoblement of E corr . The attachment of bacteria during the initial period played an important role in the ennoblement of E corr and it is believed that the carbohydrate and protein in the biofilm are reasons for this phenomenon. The double layer capacitance (C dl ) of passive metals decreased with time when immersed in natural seawater, while remained almost unchanged in sterile seawater. The increased thickness and reduced dielectric constant of C dl may be reasons.

There is a trade-off between the oxidation effect and element contamination in metal-sheathed standard platinum resistance thermometers (MSPRTs). Excessively high O 2 partial pressure causes the platinum sensor to oxidize, and excessively low O 2 leads to sensor contamination. The oxygen content in a thermometer may become unknown after a period of operation due to slow oxidation of the metal sheath and consequent loss of oxygen in the MSPRT. This can significantly affect the thermometer's performance. Our recent research has shown that a thermometer may eventually become contaminated due to a deficiency of oxygen surrounding its element. In order to research this phenomenon and improve the stability of MSPRTs, ten MSPRTs were specially manufactured for testing. In this paper, the construction of these MSPRTs is described. A series of experiments and their results are presented. Based on the experimental results, a feasible solution (having the element sealed separately from its sheath) is put forward. This solution can resolve the conflict between the oxidation effect and element contamination and improve the long-term stability of MSPRTs. The R tp and W(Al) stabilities of the MSPRTs with this new design can be as good as 1 mK and 2 mK respectively after operation at high temperature over 1000 h.