SEDL / STP / STP1049-EB / STP24064S

Effects of Electrochemical Potential on the Slow Strain Rate Fracture of 4340 Steel in a Combustion Product Residue

Daniels, RD
Professor and directorresearch assistant, School of Chemical Engineering and Materials ScienceUniversity of Oklahoma, Norman, OK

Sadarangani, AP
Professor and directorresearch assistant, School of Chemical Engineering and Materials ScienceUniversity of Oklahoma, Norman, OK

Magner, MS
Metallurgical engineer, Hercules Aerospace Co., Magna, UT

Kennelley, KJ
Senior research engineer, EXXON Production Research Co., Houston, TX

Pages: 14    Published: Jan 1990

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Abstract

Steel breech chambers in cartridge-ignition aircraft engine starters have failed in service by fracture of the chamber dome during firing. The interior surface of the dome is subject to corrosion because of accumulation of combustion products and moisture behind heat shields in the chamber. The residue collected from behind the heat shields of 25 used but still serviceable breech chambers was employed as an environment for slow strain rate tension tests of 4340 steel heat treated to the strength of the chamber. The residue contained ammonium chloride, potassium chloride, and the oxides of iron as major components. The slow strain rate tests were carried out at controlled electrochemical potentials both anodic and cathodic to the corrosion potential of -550 mV SCE (saturated calomel electrode). The tests demonstrated that the corrosion potential is at the edge of the stress-corrosion cracking potential range for the steel in this environment. Thus, stress-corrosion cracking appears to be a factor in service failures of the chambers.