Investigation of Field Failure of a Zinc Phosphate Coated UNS G61500 Disc Spring Steel Exposed to a Hydraulic Fluid Using Electrochemical Methods

摘要

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 disk spring steel. The hydraulic fluid in the affected equipment (Fluid B) was an aqueous ethylene glycol (EG) with a brand-specific 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 V_(sce)) 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 V_(sce),) 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.