Effect of Condenser Design on Stress Corrosion Cracking of Stainless Alloys in Boiling Chloride Solutions

Chiang, Y.-L., Streicher, M.A.

OnePetro 

ABSTRACT INTRODUCTION The design of condensers used to reflux boiling chloride solutions for stress corrosion tests on stainless alloys was shown to have a significant effect on time to cracking. This effect resulted from the different steady-state concentrations of oxygen produced by various types of condensers. The condenser that provided the least amount of preheating before returning the condensate to the solution established the largest concentrations of air (oxygen) in the boiling solution and the shortest times to failure by stress corrosion cracking (SCC). This was demonstrated with magnesium chloride (MgCl2) and sodium chloride (NaCl) solutions on type 304 stainless steel (UNS S30400), Carpenter 20 Cb-3 (UNS N08020), and Incoloy 825 (UNS N08825). In MgCl2, the effect of oxygen on stress corrosion depended upon the concentration of the solution. The effect was maximum in the range from 24% to 32% and decreased or disappeared at higher concentrations depending upon the alloy. These findings can be used to assess previously published data, for the design of new experiments, and for the development of new evaluation tests. Using a solution of 26% NaCl with phosphoric acid (H3PO4) to reduce the pH to 1.0 provides a more realistic method for evaluating alloys for service on plants than testing in MgCl2 solutions. Boiling solutions of chloride salts, especially magnesium chloride (MgCl2) and sodium chloride (NaCl), are used widely in investigations of the mechanism of stress corrosion cracking (SCC), the evaluation of alloys and alloy development, and in the specification of alloys for plant use. Such boiling solutions require a reflux condenser. These come in a variety of designs (e.g., Allihn) with drip tips of varying lengths and pine cone or cold finger designs (Figure 1). A variation of the Allihn condenser (modified Allihn) has been developed specifically for testing in solutions with relatively high boiling points (> 120°C), such as concentrated MgCl2 and lithium chloride (LiCl).1-2 Little attention generally has been given to the effect condenser shape may have on stress corrosion tests. In many cases, the type of condenser used has not been reported. The present investigation showed condenser design can have a major effect on stress corrosion tests in boiling MgCl2 and NaCl solutions and, therefore, that it is essential to know the type of condenser used in a given test to assess results properly and to compare results from various investigators. Information on condenser effect also can be used to resolve apparent discrepancies in the literature on stress corrosion of stainless alloys.

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