The mechanisms of corrosion inhibition of mild steel by phosphate esters in a C02 containing environment is studied by using various inhibitor solution characterization techniques (residual analysis, dynamic surface tension, emulsion tendency). Corrosion inhibition and inhibitor film persistency is monitored over time by using a rotating cylinder electrode (RCE) system and linear polarization resistance (LPR) technique. Corrosion rate-time/concentration profiles are obtained for all phosphate esters studied. Optical profilometry is used to assess surface microstructure as related to inhibitor fihn persistency. The effect of phosphate ester?s chemical structure (aromatic vs. aliphatic hydrophobic group, degree of ethoxylation, mono- vs. di-ester) on inhibitor performance is investigated in terms of their volubility, hydrolytic stability, emulsion properties, iron/calcium salt/complex formation and oil/water partitioning. A synergy between phosphate esters and irnidazolines is also investigated.


Phosphate esters are major components of oilfield corrosion inhibitors and are very effective especially at moderate temperatures or in the presence of trace amounts of oxygen.1-3 Phosphate esters are also low toxicity chemicals used as part of green corrosion inhibitors (e.g. North Sea).4>5 As effective anodic corrosion inhibitors, inhibitor products to reduce the hydrogen entry into metals.

Phosphate esters in general and ethoxylated phosphate esters in particular are used in different areas of corrosion inhibition, such as well stimulation, oil and gas production and refineries. Thus, ethoxylated tridecylalcohol phosphate esters are used in well stimulating fluids to keep Ca+ from precipitating by forming soluble salts of Ca*; their activity is thought to be due to the hydrolytic stability of the phosphate ester.8 At high temperatures, such as those experienced in refineries, phosphate esters are used to complex with Fe+ and Fe- to form insoluble iron phosphates which prevent further attack by naphthenic acids.9 A comparison of the corrosion inhibition of mono-ethanolamine salt of the phosphate ester of an ethoxylated tridecanol in a sweet test at different concentrations and a constant pH of 3.9 showed good corrosion inhibition at 50 ppm.10Testing of the same inhibitor in a flow loop showed that more inhibitor was needed with increasing shear stress to obtain the same degree of protection. 11 Water soluble ethoxylated aliphatic phosphate esters are used effectively in an environmentally acceptable corrosion inhibitor with superior environmental properties.4 Octanol derived oil soluble phosphate ester tested in a wheel test, a sparged beaker test, and a recirculating corrosion loop test showed better performance in sweet environments.*2

The excellent corrosion inhibition properties of phosphate esters obtained in these various applications makes further increase in their activities possible through structure-performance relationships and process optimization studies. Despite significant number of papers published on their applications, there is very little information on structure-activity relationship of phosphate esters or the chemistry and the mechanism behind the inhibition process. The objective of this paper is to determine the mode and the degree of inhibition of various phosphate esters studied, including most commercially available esters, by using various solution characterization techniques (e.g. residual analysis, Dynamic Surface Tension), corrosion monitoring techniques (RCE/LP~ current noise) and surface microstructure measurements (Optical Profiler).

The mechanism of corrosion inhibition of mild steel by phosphate esters under typical

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