2.1.8. Cathodic Protection 



Sacrificial zinc anodes were attached to AISI 

 Type 1015 steel to determine its effectiveness in pro- 

 viding cathodic protection to a more noble material 

 at these depths. 



The sacrificial zinc anodes were effective in 

 reducing the corrosion of the AISI Type 1015 steel. 

 They provided nearly complete protection for 123 

 days, 50% protection during 75 1 days of exposure, 

 and 30% during 1,064 days of exposure. 



2.1.9. Galvanic Corrosion 



A few galvanic couples (dissimilar metals) of AISI 

 Type 4130 and AISI Type 4140, 1 x 7-inch steel 

 strips with 1-inch-square pieces of 6061 and 7075-T6 

 aluminum alloys, AZ31B magnesium alloy, aluminum 

 bronze alloy, titanium metal, and AISI Type 308 

 stainless steel attached to them were exposed at 

 depths of 2,500 and 6,000 feet for 400 days to deter- 

 mine their compatibilities. 



After 400 days of exposure at a depth of 6,000 

 feet aluminum alloy 6061 attached to AISI Type 

 4130 steel was moderately corroded with practically 

 no corrosion of the steel; the aluminum alloy 

 7075-T6 was severely corroded under the same con- 

 ditions. Magnesium alloy AZ31B was nearly 

 completely sacrificed when attached to AISI Type 

 4130 steel, but the steel was also corroded because of 

 the insulating layer of magnesium alloy corrosion pro- 

 ducts which accumulated at the faying surfaces of the 

 two alloys. AISI Type 4130 steel was extensively 

 corroded when in contact with the aluminum bronze. 



After 400 days of exposure at a depth of 2,500 

 feet, AISI Type 4340 steel was rusted considerably 

 from being in contact with titanium metal or AISI 

 Type 308 stainless steel. 



2.1.10. Stress Corrosion 



Some of the steels were exposed in a stressed con- 

 dition at stresses equivalent to from 30 to 75% of 

 their respective yield strengths. The steels, stresses, 

 depths, days of exposure, and their susceptibility to 

 stress corrosion cracking are given in Table 7. 



One-half-inch AISI Type 4140 steel bolts, heat- 

 treated to about 175,000 psi tensile strength, failed 



during 400 days of exposure — one in the bottom 

 sediment and two in the seawater at the 2,500-foot 

 depth. Whether these failures were due to stress cor- 

 rosion or hydrogen embrittlement is not certain. 

 Bolts of such hardness should not be used in deep-sea 

 applications. 



One nickel-plated specimen of AISI Type 4130 

 steel, stressed at 127,000 psi, failed during 197 days 

 of exposure at a depth of 2,500 feet. Since no 

 unplated specimens failed, it is possible that the 

 failure was caused by the nickel plating. Hydrogen 

 absorbed into the metal during the plating process 

 could have caused hydrogen enbrittlement, which in 

 turn caused the failure. 



Some 18 Ni maraging specimens failed by stress 

 corrosion when stressed at various levels, under 

 different conditions, for different periods of time at 

 different depths. These results indicate that the stress 

 corrosion behavior of this steel is unpredictable and 

 unreliable when used at high stress levels (above 

 about 150,000 psi yield strength) for seawater 

 applications. 



The other steels were not susceptible to stress cor- 

 rosion. 



2.1.11. Mechanical Properties 



The percent changes in the mechanical properties 

 of the steels resulting from corrosion are given in 

 Table 8. 



The percent elongation of HSLA No. 5 in 

 thicknesses of 1/4 inch and 1/8 inch was decreased by 

 77 and 82%, respectively, after 400 days of exposure 

 at the 2,500-foot depth. 



The mechanical properties of AISI Type 4130 

 steel, bare, cadmium, copper, or nickel-plated were 

 affected after 400 days of exposure at the 2,500-and 

 6,000-foot depths. Cadmium, copper, or nickel 

 plating on AISI Type 4340 steel also caused decreases 

 in the mechanical properties of the steel after expo- 

 sure for 400 days at the 2,500-foot depth. 



Because of pitting corrosion the elongation of 

 AISI Type 502 (5% Cr) steel was decreased from 13 

 to 38% during all exposures at both depths, except 

 for 197 days at the 2,500-foot depth. 



The mechanical properties of the 18 Ni maraging 

 steels were, in general, adversely affected by exposure 

 at depth in the Pacific Ocean. 



