Influence of flow on the corrosion inhibition of St52-3 type steel by potassium hydrogen-phosphate

Abstract

Influence of hydrodynamic conditions on the corrosion of St52-3 type steel rotating disc electrode, RDE, in 3.5% NaCl and its corrosion inhibition using K₂HPO₄ have been studied. Results showed that by rotating the electrode in blank and inhibited solutions, corrosion current density, i_corr, increased, corrosion potential, E_corr, shifted toward more positive values and charge transfer resistance, R_ct, decreased. The inhibition efficiencies increased with electrode rotation rate. This increase was attributed to the enhanced mass transport of inhibitor molecules toward the metal surface and formation of more protective films. Little decrease of efficiencies at higher rotation speeds was probably because of the separation of protective films due to high shear stresses.

Introduction

The seawater with approximately 3.5% salinity is used by many industries such as shipping, offshore oil and gas production, power plants and coastal industrial plants. The main use of seawater is for cooling purposes but it is also used for fire-fighting, oil-field water injection and for desalination plants [1], [2], [3], [4], [5]. Corrosion and corrosion inhibition of iron and steels in NaCl containing solutions have fundamental academic and industrial importance.

Phosphorous compounds are commonly used to inhibit metals corrosion in aqueous electrolytes. Their use is relatively risk free due to their low toxicity [6], [7], [8]. For example, sodium monofluorophosphate (Na₂PO₃F) [9], calcium monoflourophosphate (CaPO₃F) [10], combination of zinc phosphate/molybdate (Actirox) and calcium ion exchange silica (Shieldex) [11] and phosphate anions [12], [13] have been used to decrease the corrosion rate of different metals in aqueous solutions.

In the industries, the equipments are often submitted to aggressive conditions (flow conditions, high temperature, etc.), involving the early apparition of corrosion. Thus corrosion and corrosion inhibition studies under hydrodynamic conditions are very important for industrial applications. However, there are few studies in literatures about the effect of hydrodynamic conditions on performance of organic and inorganic inhibitors under laminar or turbulent flow. The effect of hydrodynamic conditions on corrosion and corrosion inhibition of some metals and alloys such as copper [14], steel [15], [16], [17], [18], Cu–Ni alloys [19], [20], [21], [22], [23] and nickel–aluminum–bronze (NAB) [24] has been investigated in different media.

Ochoa et al. [25] have reported the influence of flow on corrosion inhibition of carbon steel RDE by fatty amines in association with phosphonocarboxylic acid salts. They showed that the inhibitor film formed at high rotation rate (2000 rpm) was thinner than that formed at low rotation rate (100 rpm). They also deduced that increasing the electrode rotation rate leads to an increase in the cathodic current densities, a decrease in the anodic current densities and corrosion potential shifted toward anodic direction. Cáceres and co-workers [26] have reported the electrochemical parameters and corrosion rate of carbon steel in different concentrations of un-buffered NaCl solutions under hydrodynamic condition using a superposition model. Hamdy et al. [27] have investigated corrosion and erosion–corrosion resistance of mild steel in sulfide-containing NaCl aerated solutions.

Corrosion inhibition of carbon steel using Na₃PO₄ in simulated interstitial solution of concrete (pH 12.5) contaminated by chloride ions has been studied. The influence of steel pretreatment time and electrode rotation rate on its corrosion behavior was investigated. It was found that for steel electrode immersed in inhibited solution, the inhibition efficiency decreased as electrode rotation rate increased, but when the steel specimen was pretreated in inhibitor containing solution for different immersion times, the inhibition efficiency was not significantly affected by rotation speed [28].

In our previous work, we studied the effect of hydrodynamic conditions on corrosion inhibition of steel in acidic solution using methionine as a green organic inhibitor. It was found that the inhibition performance of methionine improves with increasing the electrode rotation rate [29].

The aim of present research is to investigate the influence of fluid flow on corrosion inhibition of St52-3 type steel in NaCl solution using hydrogen phosphate ion. The St52-3 steel is widely used in most of industrial and constructional works, such as mineral processing equipments, petrochemical and oil industries, power plants, storage tanks etc. Electrochemical methods including potentiodynamic polarization and electrochemical impedance spectroscopy were performed to identify the effect of flow on inhibition efficiencies; scanning electron microscopy, SEM, technique was also used for morphological studies.