ABSTRACT

The Crevice corrosion tests were performed on UNS S31603, UNS S31703, UNS S31726, UNS S31254, UNS N08904, UNS N625, UNS N825 and UNS N276 was investigated in seawater and neutral brine solution using a multiple crevice washer assembly. PTFE multiple-crevice washers were bolted to both sides of the test specimens with PTFE bolts and nuts. The specimens were exposed to seawater flowing at a rate of 100 L/h for periods of 3000 h and 6000 h. Duplicate specimens were immersed in a plexiglass cell containing the flowing seawater at a temperature of 30°C. The results showed that all the tested coupons were susceptible to some degree of crevice corrosion attack. However, the stainless steels were the most severely affected. The degree of crevice corrosion attack for the nickel base alloys decreased as the percentage of molybdenum content in the alloys increased. Destruction of the passive layer by the concentration of chloride or acidity and reduction of hydrogen ions at the crevices is believed to be the cause of the crevice attack.

INTRODUCTION

Crevice corrosion is a form of localized corrosion that may occur within crevices or at shielded surfaces where stagnant solution is present. Crevices will always exist, for example between tubes and tube plates, gaskets, rivets, bolts, valve seats, and surface deposits ?2. Crevice corrosion can also occur under fouling, especially under hard-shell fouling in ambient temperature seawater. Crevice corrosion may also occur under deposits formed by heating seawater. Significant deposition could occur when the seawater temperature exceeds 55 - 60°C if the seawater is not chemically treated. Problems deriving from deposits are generally not encountered in condensers, which operate in desalination processes in which seawater is treated to eliminate precipitation. Interest in the application of high alloy stainless steels and nickel-base alloys for corrosion control in desalination and reverse osmosis (RO) plants has been based upon their reported improved localized corrosion resistance3-] 0. These alloys rely on a thin protective passive film on their surface to prevent corrosion. The properties of this film are a function of the specific alloy, resistance to corrosion is greater for the higher alloyed material, Under many circumstances, particularly in chloride-containing units, this passive film can break down resulting in localized corrosion. Therefore, crevice corrosion resistance is a major factor in determining which alloy should be selected for use in desalination and RO plants.

The present paper describes the crevice resistance of some commercial stainless steels and nickel- base alloys under aerated conditions in RO seawater solution using the multiple crevice assembly tests.

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