Elsevier

Corrosion Science

Volume 44, Issue 4, April 2002, Pages 717-728
Corrosion Science

Rhodanine azosulpha drugs as corrosion inhibitors for corrosion of 304 stainless steel in hydrochloric acid solution

https://doi.org/10.1016/S0010-938X(01)00100-7Get rights and content

Abstract

The effect of rhodanine azosulpha drugs on the corrosion behaviour of 304 stainless steel in 1.0 M hydrochloric acid solution as corrosive medium has been investigated using weight loss and potentiostatic polarization techniques. Some corrosion parameters such as anodic and cathodic Tafel slope, corrosion potential, corrosion current, exchange current densities, surface coverage and inhibition efficiency were calculated. The polarization measurements indicated that the inhibitors are of mixed type and inhibit corrosion by parallel adsorption on the surface of steel due to the presence of more than one active centre in the inhibitor molecule. The adsorption is obeyed Langmuir adsorption isotherm. The activation energy and thermodynamic parameters were calculated at different temperature.

Introduction

Type 304 stainless steel (304SS) has found wide application in a variety of industries. It is covered with a highly protective film of chromium oxyhydroxide and is resistant to corrosion in many aggressive environments. Hydrochloric acid solution, however, readily attack 304SS. It is possible to reduce the corrosion rate to safe level by adding inhibitors. Most of the well-known inhibitors investigated for corrosion of steel in acidic solution are heterocyclic compounds [1], [2], [3], [4], [5], [6], [7]. For these compound it is clear that the adsorption on the metal substrate is the initial step in inhibition. The adsorption inhibitor is related to the presence of heteroatoms such as nitrogen, oxygen, phosphorous and sulphur and long carbon chain length as well as triple bond or aromatic ring in their molecular structure. Generally a stronger co-ordination bond cause higher inhibition efficiency (IE) [8].

The present investigations aim to study the effect of four rhodanine azosulpha drugs as corrosion inhibitors on the corrosion of 304SS in 1 M HCl using two methods (weight loss and polarization measurements). The effect of temperature on the dissolution of 304SS in 1 M HCl containing 10−3 M of the inhibitors used was also studied and some thermodynamic parameters were computed.

The following four rhodanine azosulpha drugs were prepared by a method described earlier [9].Compound I: 5-sulphadiazineazo-3-phenyl-2-thioxo-4-thiazolidinoneCompound II: 5-sulphamethazineazo-3-phenyl-2-thioxo-4-thiazolidinoneCompound III: 5-sulphadimethoxineazo-3-phenyl-2-thioxo-4-thiazolidinoneCompound IV: 5-sulphamethoxazoleazo-3-phenyl-2-thioxo-4-thiazolidinone

Section snippets

Experimental method

The composition of the stainless steel type 304 is in wt.% 0.045C, 0.35Si, 1.64Mn, 0.17Co, 0.026P, 0.014S, 18.0Cr, 8.8Ni, 0.19Mo, 0.16Cu and 0.044N. The 304SS strips of 1 cm2 size were used for weight loss measurements. For potentiostatic studies, a cylindrical rod embedded in araldite with an exposed bottom surface area of 1 cm2 was used. The electrode was polished with different grades of emery paper, degreased with acetone and rinsed in distilled water.

Weight loss measurements were carried

Weight loss measurements

304SS cupons were exposed for 24 h in naturally aerated 1 M HCl with and without addition of the inhibitor. From the value of weight loss the IE at each inhibitor concentration was calculated using the equation:IE=1−waddwfree100where wadd and wfree are the weight loss of the metal in presence and absence of the inhibitor, respectively. The results are collected in Table 1. It is obvious that, the IE increases with increasing the inhibitor concentration whereas decreases in the following order:

Thermodynamic parameters

The Langmuir adsorption isotherm may be formulated ((C/θ)=(1/K)+C). The plot log (C/θ) vs log C found to be linear for each additives (Fig. 4). The equilibrium constants (K) for adsorption desorption process for each compound can be calculated from the reciprocal of the antilogarithm of the intercept. The K values were listed in Table 3.

The thermodynamic parameters free energy (ΔG) and entropy (ΔS) of the investigated compounds can be calculated from the thermodynamic relationsK=exp−ΔGRTΔG=ΔH−TΔS

Conclusions

1. Azosulpha drugs compounds have proved to be efficient inhibitors for corrosion of 304SS in 1 M HCl solution. These inhibitors act as mixed type inhibitors and the IE was found to increase by increasing the inhibitor concentration and decreasing of temperature.

2. Compound III which contains two methoxy groups give the higher inhibition efficiency compared to the other compounds I, II and IV.

3. The IE obtained from the polarization measurements show good agreement with those obtained from

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