Elsevier

Corrosion Science

Volume 48, Issue 6, June 2006, Pages 1404-1412
Corrosion Science

The effect of molybdate concentration and hydrodynamic effect on mild steel corrosion inhibition in simulated cooling water

https://doi.org/10.1016/j.corsci.2005.06.009Get rights and content

Abstract

The effects of two important factors namely concentration and hydrodynamics on the inhibition efficiency of molybdate were investigated using potentiodynamic polarization and electrochemical impedance spectroscopy tests. Results showed that molybdate was capable of inhibiting the corrosion of mild steel in simulated cooling water. The inhibition efficiency of molybdate was increased with increasing both its concentration and water circulation velocity. These two factors seem to promote the adsorption of molybdate and oxygen ions on the metal surface, leading to the formation of a protective layer with a greater charge transfer resistance and lower permeability. A model is also proposed which facilitates the probable mechanism of inhibition.

Introduction

The corrosion of steel equipment in contact with circulating water is one of the major problems in cooling systems [1]. The steel equipment undergo corrosion because of the corrosive nature of the circulating water originating from the existance of dissolved oxygen and concentration of aggressive ions such as chloride and sulphate [2], [3]. The corrosion problems are particularly intensified if seawater is allowed into the system to act as the coolant or high concentration cycles are used. Application of corrosion inhibitors is one of the preventative methods which has been reported to be helpful in cooling systems [4], [5], [6], [7]. Molybdate is one of the most versatile corrosion inhibitors which has been proved to be capable of controlling the corrosion of different metals and alloys in a variety of corrosive media [8], [9]. The successful application of molybdate to mitigate the corrosion has been reported in automobile cooling systems [10], paints [11], cooling towers [12], metal forming fluids [13], [14], lubricants, boilers, metallic glasses, acids and concrete [8]. Besides steel, titanium in acidic media [15] and zinc in NaCl solutions [16] have been protected against corrosion using molybdate. Many other researches [17], [18], [19], [20], [21] have concentrated upon the synergism between molybdate and other organic and inorganic compounds for corrosion reduction in cooling systems. Different operational parameters such as cooling water pH, inhibitor concentration, water circulation velocity and the concentration of other ions present in water may have a considerable influence on molybdate inhibition efficiency in cooling waters [22], [23], [24].

In this research the behaviour of molybdate as an inhibitor for controlling the corrosion of mild steel in simulated cooling water was investigated. The influence of molybdate concentration and hydrodynamic effect on its inhibition efficiency was evaluated using potentiodynamic polarization and electrochemical impedance spectroscopy.

Section snippets

Experimental procedure

The mild steel St37 specimens were used as the base metal. Potentiodynamic polarization experiments and impedance spectroscopy tests were conducted in a non-aerated simulated cooling water with the chemical composition presented in Table 1. All the experiments were performed at room temperature. Sodium carbonate, sodium bicarbonate, calcium chloride and sodium sulphate were used for preparation of the simulated cooling water and its pH was adjusted at 8.2 ± 0.1. Specimens with the exposed area of

Results and discussion

The anodic polarization curves for mild steel in simulated cooling water at different concentrations of molybdate are shown in Fig. 1. To eliminate any doubt about IR drop in the electrolyte due to its dilution, IR drop compensation was done using current interruption method. However, since the results of the effect of corrosion inhibitor were comparative, the difference between polarization curves with and without IR drop compensation was not significant. It is seen that with increasing

Conclusions

  • 1.

    Molybdate shows to have the capability of reducing the corrosion rate of mild steel in simulated cooling water.

  • 2.

    Increasing molybdate concentration has a positive influence on its performance as a corrosion inhibitor in cooling systems but it has an optimum limit depending on the aggressive ions concentration.

  • 3.

    Hydrodynamic effect can improve the inhibition efficiency of molybdate through the promotion of oxygen and molybdate movement toward the electrode surface but higher electrode rotation rates

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