The malfunction of a hydraulic system on an offshore production platform was traced to environmentally
assisted cracking (EAC) of a zinc phosphate conversion coated UNS G61500(1) disk spring steel. The
hydraulic fluid in the affected equipment (Fluid B) was an aqueous ethylene glycol (EG) with a brandspecific
proprietary inhibitor package and it caused EAC of the equipment at approximately 50°C. Other
hydraulic systems (Fluid C) using a different inhibitor package had not experienced any similar failures
as of the incident field investigation.
Hydrogen embrittlement (HE) was considered as one possible cause of the observed cracking. In this
work, multiple electrochemical tests were conducted to characterize the electrochemical behaviors of the
conversion coated disk spring steel in a benchmark EG hydraulic fluid without additives added, Fluid B,
and Fluid C.
It is found, in 50°C-Fluid B, the hydrogen evolution reaction (HER) threshold potential for the disc spring
steel was determined as -0.7 Vsce, more positive than that obtained (-1 Vsce) from 50°C-Fluid C,
suggesting HER on the steel immersed in Fluid B was more feasible than in Fluid C. The well-coated
steel in Fluid B showed open circuit potential (OCP) higher than the respective HER threshold potential
(-0.7 Vsce,) at 50 °C, indicating the HER and the related HE of the steel may not occur. However, If the
conversion coating degraded and lost corrosion protective property, the OCP could drop below the HER
potential and possibly induce HE.