A world-wide test program was undertaken by ASTM Task Group Gl.09.02.03 to assess the relative corrosivity of seawater at 14 test sites. Aluminum alloy 5086 (UNS A95086), 90/10 copper-nickel (UNS C70600), and copper-bearing carbon steel (UNS K01501) test specimens were prepared at one location, shipped to the various sites, and returned to the original location for final evaluations. Results obtained through five years of testing indicate that corrosion behavior was generally within the limits of previously published results. The results show that while seawater is a ubiquitous environment, and quite similar in terms of chloride content and pH, the corrosivity is site- specific, and likely to be influenced by a myriad of other factors such as temperature, dissolved oxygen concentration, flow, degree of fouling, bacterial activity, pollution, etc. All of these factors are themselves often interrelated. The cooperation of all program participants has contributed much toward accomplishment of the objectives. More frequent monitoring of seawater variables at an exposure site is always helpful in better interpretation of the results of corrosion tests performed there. All of the 0.5 through 5-year exposure data are presented here; any typographical errors in the 0.5 through 3-year exposure data published previously have been corrected.

A world-wide test program was undertaken by Task Group G 1.09.02.03 to assess the relative corrosivity of seawater at 14 test sites. Aluminum alloy A95086, copper-nickel alloy C70600, and carbon steel alloy KOI501 specimens were prepared at one location, shipped to the various sites, and returned to the original site for final evaluations. Results obtained through three years of testing indicate that average corrosion behavior was generally within limits of previously published results. Individual site characteristics have been identified, however, that can have a profound effect on test results. Even when the ASTM standard test method was prescribed, variations affecting corrosion results became evident. In reality there is no natural seawater environment, as identified to date, in which to test materials. The final five year results are yet to be collected, but the cooperation of all program participants has contributed much toward accomplishment of the objectives. Still, more frequent and broader monitoring of sea-water variables at the exposure sites would assist in interpreting corrosion results.

The ultimate successful utilization of stainless steels and related alloys in marine and other chloride containing environments will depend on a more complete understanding of the factors affecting crevice corrosion. To this end, numerous test methods have been devised to investigate the effects of various metallurgical and environmental parameters. While not totally ignored in past research, more recent investigations have also emphasized the importance of crevice geometry considerations. For example, mathematical modelling of chemistry changes in the crevice electrolyte indicates a strong dependency on crevice gap and crevice depth dimensions. In addition, the bulk environment chloride level will affect the ultimate crevice electrolyte pH and chloride level. Accordingly, a given stainless steel may encounter a range of crevice solutions of varying pH and chloride level dependent upon the crevice geometry and bulk environment conditions encountered. Two experimental approaches for studying crevice corrosion are presented. A remote crevice assembly and a compartmentalized cell have been used to study both the initiation and propagation phases of crevice corrosion. Both methods make use of two specimen components consisting of physically separated but electrically connected anodes (crevice portion) and cathode members. Accordingly, corrosion potential and current data along with conventional mass loss and penetration data can be gathered. A significant feature of these methods is that neither requires the application of current from an external source. Both initiation and propagation of crevice corrosion occur spontaneously. The two techniques differ slightly in that the remote crevice assembly requires creation of a physical crevice on the anode while the compartmentalized cell approach uses a nonoccluded anode in a simulated crevice electrolyte. With the compartmentalized cell, crevice electrolytes are simulated based on the predictions of a mathematical model of crevice corrosion. Results from recent tests investigating the effects of variations in crevice tightness, bulk solution composition, and crevice solution pH are described in the present article. Remote crevice assembly research has focused on efforts to provide a consistent and reproducible condition of crevice tightness on the anode. The compartmentalized cell research has dealt with the effects of changes in both bulk environment conditions and crevice solution pH. Insights gained on the relative influence of these various factors on crevice corrosion lend credence to the testing concepts embodied by both the remote crevice assembly and the compartmentalized cell.

Thermomechanical treatment consisting of carbide stabilizing aging of cold worked materials followed by low temperature recrystallization heating (SAR process) made the standard stainless steels highly resistant to intergranular corrosion and cracking in otherwise susceptible environments. After various reheating thermal exposure several kinds of IGC and IGSCC susceptibility tests were made on the optimized SAR treated materials to see the critical conditions for the microstructural stability. The microstructural stability was maintained up to 825C and 875C in Types 304 SAR and 316 SAR respectively. SAR treated materials showed no IGC and IGSCC susceptibility under the SSRT in 288C oxygenated pure water, under the constant load tests in boiling 20% MgC12 solution and in the hot corrosion environment. In addition, the improved mechanical strength of SAR treated materials relative to the conventional materials is one of the advantages to maintain high strength in those environments.

The corrosion resistance of graphite/aluminum (Gr/Al) and silicon carbon/aluminum (SiC/Al) metal matrix composite (MMC) materials and composite corrosion protection methods were investigated in marine environmental exposures ranging from 30 to 365 days. Accelerated corrosion of the Gr/Al resulted from galvanic interactions between the graphite fibers and the aluminum matrix. Corrosion of the SiC/Al was due to pitting, which was oriented at the silicon carbide-aluminum interfaces in the discontinuous forms of silicon carbide reinforcement. Differences in surface foil alloy and composite structure for the Gr/Al or type of silicon carbide reinforcement for the SiC/Al did not affect the materials' overall corrosion resistance. A variety of corrosion control coatings was found to be suitable for protection of the MMC, including organic and thermal-sprayed coatings.