Small variations in aluminum can change the phases which develop. Alloy A (9.4% Al) typically consists of Widmanstatten α plus Martensite and Bainite interdispersed between the Widmanstatten α. Alloy B (8.1%) usually has an equiaxed α microstructure with isolate equiaxed regions of Martensite and Bainite. The Martensite/Bainite in both Alloys A and B can be transformed to eutectoid by a subsequent tempering heat treatment, or by furnace cooling (FC).
Based upon only one specimen of two compositions in each of five conditions of heat treatment, it is difficult to reach broad conclusions in regard to seawater jet impingement resistance, particularly in such a complex alloy system. It has been noted that increasing the velocity from 15 to 30 fps has a much greater effect on the diameter of the impingement zone than on maximum depth of attack in Alloy A in both the AC+T and OQ+T conditions. However, as identified here-in and in the quiescent test, other forms of localized corrosion may affect these materials differently. Nevertheless, it can be concluded that, based upon physical appearance, mass loss, and maximum depth of attack after being subjected to seawater impingement testing at 15 and 30 fps, that Alloys A and B as well as the control, Alloy C , all exhibited similar resistance to impingement attack. This is somewhat surprising in that the aluminum content of Alloy B falls outside the lower limit for aluminum (8.5%) listed in UNS C95800. In regard to the effect of heat treatment on mass loss and maximum depth of attack after seawater impingement testing, no apparent correlation is seen in either alloy. This uniformity in jet impingement resistance is also surprising in view of the markedly different microstructures observed in Alloys A and B. In the previous quiescent seawater corrosion evaluation of these same materials, Alloy A exhibited better corrosion resistance relative to Alloy B (8.1% Al), because the latter alloys suffered from selective phase attack. In summary, it can be concluded, that neither variation in alloy composition of the tested materials, nor heat treatment and resulting microstructure, have a significant influence on seawater jet impingement resistance in the range of 15 to 30 fps.
Support and funding, was provided by the Copper Development Association Inc. (CDA). We would also like to thank Professor Paul Howell and K.C. Meinert of Pennsylvania State University for assistance in identification of phases in the photomicrographs. In addition, we wish to thank the staffs of the LaQue Center for Corrosion Technology and the CDA for logistical support.
Paper Number 05223 reproduced with permission from Corrosion/2005 Annual Conference and Exhibition, Houston, Texas