Pitting and Deposits with an Organic Fluid by Electrolysis and by Fluid Flow

Abstract

Electrolysis experiments were performed with iron electrodes in phosphate ester fluids. Pitting corrosion occurred and films were deposited on the electrodes. The effect of various additives was investigated and a model involving a threshold current density for pitting was formulated. The relationship of the results to corrosion by electrokinetic currents is studied. The occurrence of eleetrokinetically produced corrosion is discussed. This paper describes some electrochemical experiments which were performed in phosphate-ester based hydraulic fluids. The experiments were associated with the discovery of a new wear mechanism in steel hydraulic-control valves. The wear mechanism is a corrosion reaction driven by electrokinetic currents associated with the flowing fluid and has been described in detail elsewhere (1). The present paper is concerned primarily with the associated electrochemistry of the phosphate ester fluids and its relationship to the wear mechanism.A primary objective of the experiments was to determine if corrosion could be produced by purely electrochemical means, without the fluid flow present in the hydraulic valves. The experiments were performed using phosphate ester fluids (2), whose properties are listed in Table I.Parallel-Electrode Cells An experiment was conducted with 2 • 12 cm parallel pure-iron sheet electrodes, separated by 0.3 cm, and immersed in 850 cc of continuously stirred phosphate ester fluid in a Pyrex beaker.For an applied potential of 500V the initial current was 2 mA and decayed over the 313 hr of the test to 0.8 mA. This represented a range of current densities of 0.3-0.8 mA/cm 2 based on the facing areas. No corrosion of the electrodes was observed but deposited films were observed.The electrodes were washed with acetone and films that had formed were scraped off. Upon returning the electrodes to the cell, the current was about 11% greater for a given potential than the current at the beginning of the run. In this and other experiments the anode film had the high resistance. Fluid conductivity, measured in a conductivity cell, was found to have increased slightly in this experiment although it decreased in other experiments. " Electrochemical Society Active Member.

Electrokinetic-Current Corrosion

“…The cause of the initial decrease on with flow rate is not known for sure, but it may be due to decrease in 0 by activation polarization in the upstream and downstream regions where wall current densities occur and to formation of deposits. 3 The effect of additions of FC-98 to SAE 1A on wall current as a function of flow rate in the wall current device is shown in Fig. 8.…”

“…2 Subsequently, further work was published on electrochemical pitting and deposit formation in phosphate ester fluids. 3 At the time that this research was done, two possible additive fixes to the fluids were recommended: addition of an easily oxidizable organic sacrificed instead of corroding the steel, and addition of an ionically conducting species thought then to give back conduction in the fluid through the orifice rather than corrosion. Both were tried by the fluid vendors.…”