Stability of titanium tubes towards acid wash

• Published in: Journal of electroanalytical chemistry and interfacial electrochemistry 1986-12, Vol.214 (1), p.567-587
• Main Authors: Shams El Din, A.M., El Shayeb, H.A., Abd El Wahab, F.M.
• Format: Article
• Language: English
• Subjects: Applied sciences; Corrosion; Corrosion tests; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Materials science; Metals, semimetals and alloys; Metals. Metallurgy; Other techniques and industries; Physics; Specific materials
• ISSN: 0022-0728
• DOI: 10.1016/0022-0728(86)80125-5

The technique of open-circuit potential measurement was employed to study the stability of titanium in solutions of different concentrations of HCl, H 2SO 4, H 3PO 4, HNO 3, HClO 4, H 2CrO 4, acetic and citric acid. Two modes of potential change were recorded. The first was noted in solutions of HCl, H 2SO 4 and H 3PO 4 of moderate to high concentrations. The potential drifted with time towards more negative values to yield a definite arrest, identified as the Flade step of Ti. The length of the arrest varied with the acid concentration according to τ = Ac − n , where A and n are contacts depending on acid type. The increase in solution temperature reduced the length of the arrest and shifted the activation of the metal to lower acid concentrations. The Flade arrest of Ti was analyzed theoretically using the equation of chronopotentiometry of irreversible electrode reactions. The results of the analysis suggested the reductive dissolution of the air-formed TiO 2 following a 2 e − rate-determining step. The potential of the Ti electrode in solutions of NHO 3, HClO 4, H 2CrO 4, acetic and citric acid changed with time towards less negative values, denoting film repair and thickening. This occurred through the action of either oxidizing agents or through the specific adsorption of the acid anions on the surface of the air-formed oxide. Potential-log time curves were constructed and allowed the rates of oxide thickening to be computed. Breaks in some of the curves were noted and a probable explanation for these is presented. Addition of minute concentrations of oxidizing agents to a corrosive acid solution prevented the activation of Ti completely. This was demonstrated with the HNO 3/HCl and the K 2Cr 2O 7/H 2SO 4 systems. Adsorption-type organic inhibitors were without effect in retarding the activation of Ti electrodes, unless they possessed marked oxidizing properties. this was demonstrated with cupferron additions. Inhibition at high concentrations of polyethyleneimine was the result of acid neutralization rather than corrosion inhibition. The effect of time of treatment in synthetic sea water on the rate of TiO 2 formation was investigated systematically. The time of activation varied with that of treatment according to a double logarithmic relationship. Theoretical analysis of the activation curves in HCl and H 2SO 4 gave parallel straight lines with comparable slopes. Dissolution of the electrolyte-formed oxide was governed by a 1 e − rate determining