When designing for corrosion resistance, the cost of the material must always be weighed against the alloy?s corrosion resistance. Though Ni-Cr-Mo alloys offer excellent corrosion resistance, the cost of these alloys can be quite high. Austenitic stainless steels, though attractive from a cost standpoint, can fail in service leading to downtime and other subsequent costs. An alloy that can bridge the gap between these two alloy groups would deliver nearly the corrosion resistance of the Ni-Cr-Mo alloys without the upfront cost would be desirable. Alloy 27-7MO has been shown to achieve this goal in many applications. This paper will discuss both laboratory and plant corrosion testing in applications where cost savings can be achieved by either eliminating failures or decreasing upfront expenditures. Environments and components examined will include heat exchangers, phosphoric acid, sulfuric acid, and other relevant applications.
Many materials are available which exhibit excellent resistance to general corrosion in a given medium (see Table 1 for compositions of several tested alloys). Unfortunately failure of a component can occur very quickly by a localized attack (pitting and/or crevice corrosion). By examining the Pitting Resistance Equivalency Number (PREN) and results from corrosion tests that focus on the localized corrosion resistance of candidate alloys, a better understanding of material performance can be achieved. Tests have been performed on a variety of alloys in various seawater-base environments, sulfuric acid environments, and phosphoric acid environments and suggest possible alternatives to the cost of some high performance alloy candidates and the high costs of downtime and repair associated with inadequate materials. Unlike testing in pure reagent grade acids, other corrosive media like chlorides and fluorides have been added to simulate aggressive conditions that might be found in actual chemical processing service.
For heat exchangers, often the most extreme conditions are encountered not with the product, but with the cooling or heating fluid. Seawater, as an abundant cooling medium in marine and coastal operations is often used in heat exchangers. Therefore heat exchangers must utilize materials that can resist both general and localized corrosion by seawater. In addition to seawater, many heat exchangers use small additions of chlorine to control the problems associated with bacteria and other marine life. Thus, not only seawater, but seawater with additions of chlorine is used, which can significantly increase the corrosivity of the environment. Conventional stainless steels like 304 and 316 have limited resistance to this environment and are very susceptible to general attack, as well as pitting or crevice corrosion and even chloride-induced stress corrosion cracking. Often, commercially pure (C.P.) grades of titanium are used for their excellent resistance to these environments. While titanium is a good technical choice, its price or especially its availability often impedes its use and consideration as a practical material of construction. Other Ni-base corrosion resistant alloys (CRA?s) like UNS N10276, UNS N06022, UNS N06625 and UNS N06686 have been qualified and used in marine heat exchangers due to their excellent resistance. Unfortunately, raw material price volatility, especially that of nickel and molybdenum, leads to higher prices on these CRA?s that might limit their use.
Conventional 6% molybdenum super-austenitic stainless steels like UNS N08367 or UNS N08926 have also been considered for this service. These alloys offer greatly improved resistance to localized corrosion when compared to standard stainless steels, but
