ISSN 2070-2051, Protection of Metals and Physical Chemistry of Surfaces, 2015, Vol. 51, No. 4, pp. 693–700. © Pleiades Publishing, Ltd., 2015. 693 1 INTRODUCTION Corrosion induced damage on metallic alloy and stainless steels are a major problem to industrial pro- duction and operating facilities and equipments due to metal degradation and insidious attacks such as pitting corrosion. These results in production loss, costly maintenance and high operating cost. In addition, leaks of process fluids may also lead to unacceptable health and safety hazards, risk of damage to the envi- ronment, as well as the associated clean-up costs. All of these expenditures can have a huge financial impact to the operating company. The corrosion resistance of stainless steel is due to the formation of a strong adher- ent, compact and continuous oxide film of chromium on the steel surface. In aqueous acid solutions the pas- sive film is basically of duplex nature, consisting of a chromium-rich inner barrier oxide layer and iron-rich outer deposited hydroxide or salt layer [1–10]. The passive films are vulnerable to localized attack espe- cially at regions of flaws or defects by corrosive ions such as chlorides, sulphates, thiosulphates, bromides etc eventually leading to pitting corrosion and inten- sive structural damage. Breakdown of the passive film from pit initiation occurs at a critical potential called pitting potential, E pit . This is one of the important parameters that typify the susceptibility of stainless steel to pitting corrosion. Significant number of attempts has been done to 1 The article is published in the original. investigate the pitting of stainless steel [11], however most of the theoretical assumptions suggests that chlo- ride ions diffuses through the passive films before film breakdown upon reaching the metal/film interface. The pitting processes can be divided into two, the first passive film breakdown and the consequent metal sub- strate dissolution. Generally pitting corrosion is invariably the most destructive form of corrosion espe- cially due to difficulty in predicting its occurrence. As complexity of the corrosive environment increases, the accuracy of the determinant parameters for pitting corrosion evaluation decreases. The corrosion process occurs in a series of steps beginning with the initiation stage which itself is a product of various phenomena that are mainly properties of the solution and those of the metal [12–17]. Aqueous sulphuric acid solutions containing chlo- rides have been employed in the investigation of the passivation behaviour of stainless steel [18–21], due to the combined action of the aggressive ions and high reproducibility of the pitting corrosion parameter (the pitting potential E pit , the repassivation potential E rp ), thus providing standard environment for pitting corro- sion evaluation. Inhibitor performance is based its capacity to anodically influence the pitting potential higher potential [22]. Thus this investigation aims to evaluate the pitting corrosion inhibition of 2-Amino- 5-ethyl-1, 3, 4-thiadiazole on the electrochemical behaviour of austenitic stainless steel in dilute sulphu- Pitting Corrosion Inhibition of Type 304 Austenitic Stainless steel by 2-Amino-5-ethyl-1,3,4-thiadiazole in Dilute Sulphuric Acid R. T. Loto a,b and C. A. Loto a,b a Department of Mechanical Engineering, Covenant University, Ota, Ogun State, Nigeria b Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, Pretoria, South Africa e-mail: tolu.loto@gmail.com Received March 20, 2014 Abstract—The electrochemical behaviour and inhibitor protection of 2-amino-5-ethyl-1,3,4-thiadiazole (TTD) on the pitting corrosion of austenitic stainless steel (type 304) in dilute sulphuric acid solution con- taminated with recrystallised sodium chloride was evaluated with the aid of potentiodynamic polarization method. TTD greatly reduced the corrosion rate of the steel with a corrosion inhibition efficiency ranging from 88.99–87.36%. The corrosion potential, pitting potential, repassivation potential, nucleation resis- tance, passivation range and repassivation capacity measurements and potentiodynamic studies were applied to assess the steel’s pitting resistance characteristics and behaviour in the acid chloride media. Results showed that pitting potential values increased with addition of TTD compound in conjunction with increase in the passivation range which strongly indicating increased electrochemical resistance to pitting corrosion. DOI: 10.1134/S2070205115040231 PHYSICOCHEMICAL PROBLEMS OF MATERIALS PROTECTION