Stainless steels have been extensively used in automotive, industrial, electronics, etc., applications. Iron (Fe) and chromium (Cr) are the main elements with weight percentage contribution of 60-75% and 10-25% respectively. Other elements, such as Ni, Co, Mo, Mn, C etc. are also present with variable concentrations. The purpose of this work is to use different electrochemical and surface techniques to study the corrosion behavior of stainless steel type 316 (percent composition of different chemical elements are listed in table 1) in acid media in presence and absence of different thiophene derivatives (list of inhibitors in figure 1). Moreover, other important goals were to study the effect of adding chloride ion to the acidic media on the corrosion behavior of stainless steel, protection efficiency of ihibitors studied, and to determine the temperature coefficient and the adsorption isotherm of the inhibitor on the stainless steel type 316.
Electrochemical techniques such as potentiodynamic polarization, Tafel experiments, polarization resistance and electrochemical spectroscopy were used to evaluate the effect of the inhibitors on the corrosion of stainless steel type 316. Surface analyses were employed to study the surface morphology and structural analysis of the surface using scanning electron microscope(SEM), Fourier Transform infrared(FT-IR), and x-ray diffraction techniques EDAX.
The results showed distinct effects for the different inhibitors used that depend on the molecular structure and the electron density on the sulfur atom of the thiophene ring. The order of inhibition efficiency was 2-thiophene carboxylic hydrazide \u3e 2-thiophene carboxylic acid \u3e 3- thiophene caroxaldhyde \u3e 2-acetyl thiophene. It was concluded that the inhibitors studied were of the mixed type.
The adsorption pattern for the inhibitors at the stainless steel surface followed a Langmuir isotherm model. The thermodynamic parameters of adsorption were calculated. It was concluded that a thin layer of inhibitor is formed at the surface of steel preventing the corrosion of the specimen in the acid medium. It was also suggested that anchoring of the sulfur atom of the thiophene ring to the surface of the stainless steel takes place that allowed a blanket of the inhibitor molecule to cover the surface.
Surface reflectance FT-IR proved the adsorption of the inhibitor molecule at the stainless steel surface. Scanning electron microscopy showed that the presence of inhibitor protected the surface of the stainless steel against pitting in chloride-containing sulfuric acid electrolyte
