Abstract
Crevice, pitting, galvanic, intergranular and uniform corrosion occur at the Mk 26 Guided Missile Launching System (GMLS) guide arm assemblies due to inherent design deficiencies and normal interaction with the operating environment. Corrosion prevention in the guide arm assembly hydraulic access cover areas, which primarily suffer from pitting, crevice and intergranular attack was studied. 1008, 1018, ASTM A514 Grade B and ASTM A514 Grade E specimens were subjected to salt-spray, inhibitor and cathodic protection tests to evaluate various corrosion control measures. 3% by weight sodium nitrite, 0.4% potassium chromate, 1% disodium phosphate, 0.8% sodium silicate and 5% calcium benzoate aqueous inhibitor solutions were tested, with Port Hueneme, CA sea water as the solvent. Sodium nitrite proved to be the most effective in totally immersed and waterline conditions. However, since the test solutions were not changed nor inhibitors replenished during the test period, the decrease in concentration of this inhibitor with time resulted in severe crevice corrosion at localized areas in the waterline zone. The sodium silicate followed by calcium benzoate solutions provided the best long-term protection at the waterline. All of the inhibitors tested provided good protection under totally immersed conditions. Wash primer, zinc chromate primer, epoxy polyamide, aliphatic isocryanate urethane and silicon alkyd paints in various combinations were evaluated in salt - spray tests. A wash primer + zinc chromate + epoxy polyamide + aliphatic isocryanate paint system best withstood these tests. After long term atmospheric exposure, subsequent SEM analysis showed that silicon alkyd paints become brittle with age, cracking radially from local breaches in the coating. Cathodically polarizing to -780 mv vs. S.C.E. proved adequate to protect 1008, 1018, ASTM A514 Grade B and A514 Grade E steels immersed in still sea water under laboratory conditions. In terms of practicality and invokeability aboard US Navy vessels, cathodic protection and use of inhibitors for Mk 26 GMLS corrosion control cannot be used. Rather, changes in design of future productions, better paint (coating) systems, and periodic removal of aggressive electrolyte ions are the only viable measures.
