The Effect of Halide Ions on the Anodic Passivation of Titanium*

By Susumu Morioka** and Akimi Umezono***

The effect of halide ions on the anodic passivation of titanium in acid, neutral and alkaline solutions has been studied with the following results: (1) In the presence of fluoride, a severe attack of titanium occurs in an acid solution owing to the chemical dissolution of the oxide film by undissociated HF. However, titanium is not attacked in neutral and alkaline solutions. (2)Chloride ion does not prevent the passivation of titanium owing to a very high discharge overvoltage of chloride 'ion on titanium. (3)Bromide ion discharges from acid and neutral solutions at a high overvoltage, causing a pitting corrosion in a neutral solution. (4) Iodide ion also shows a high discharge overvoltage in acid and neutral solutions, but not in an alkaline solution below the discharge potential of hydroxyl ion. No pitting corrosion occurs by the discharge of iodide ion. (5) The corrosion resistivity of titanum in halide-bearing solutions is due to its larger affinity with oxygen than with halogen. Therefore, in aqueous solutions the amount of the dissolved oxygen, hydroxyl ion, or oxidizing agent has a significant effect on the corrosion resistivity of titanium. No pitting corrosion takes place unless a considerable external electrolytic current is applied, and the pitting corrosion is usually prevented under natural conditions. (ReceivedJuly 7, 1964)

I. Introduction Table 1 Chemical composition of the titanium Titanium exhibits remarkable high resistance a- specimen. gainst corrosion even in the presence of halide ions. In the present report, the effects of halide ions on the anodic passivation of titanium are described. The experimental results were compared with those on platinum.

urement of polarization curves was started 30 min II. Experimental Materials and Methods after immersion of the specimen. For some specimens, The chemical composition of the titanium specimen cathodic treatment was made for 30 min at the current used is shown in Table 1. density of 1mA/cm2 immediately after the immer- Each specimen was kept in air 30 sec after degreas- sion. The current was then cut off and the measure- ing and then placed in the test solutions. The meas- ment of polarization curves was made 5 min later. * This paper was published in Japanese in the Journal of The acid solution was prepared by adding 0.1mol the Japan Institute of Metals., 23 (1959), 185. potassium halide in a liter of 2N sulphuric acid solu- ** Department of Metallurgy , Faculty of Engineering, Tohoku tion. The neutral solution was the potassium halide University, Sendai, Japan. 0.1mol/l aqueous solution, and the alkaline solution *** Central Research Laboratory , Fuji Iron & Steel Co., Sagamihara. Japan. was the 1N potassium hydroxide solution and the same

Trans. JIM 1964 Vol.5 Susumu Morioka and Akimi Umezono 199

solution to which 0.1mol potassium halide per liter slight difference in the acid and neutral solutions was added. owing to the unstability of iodide ion in the solutions. In the alkaline solutions containing potassium fluoride, chloride or bromide the Tafel line for the evolution of III. Experimental Results oxygen was observed, while the solution containing 1. Platinum potassium iodide showed the Tafel line for the dis- Fig. 1 shows anodic and cathodic polarization curves charge of iodide ion. of platinum. Since platinum is not attacked in these The cathodic polarization curves indicate, of course, solutions, the potentials represented by the Tafel lines the Tafel line for the discharge of hydrogen ion at the indicate oxygen, halogen or hydrogen evolution poten- potential corresponding to the hydrogen ion concentra- tials, respectively. tion of each solution used.

Fig. 1 Polarization curves for platinum in naturally aerated solutions at 20℃.

Fig. 2 Anodic polarization curves for titanium in naturally aerated 2N-H2SO4 solution and that contining 0.1mol/l potassium halide at 20℃.

The anodic polarization curves obtained in the 2N 2. Titanium sulphuric acid solution and the solution containing With titanium the situation is far less simple than potassium fluoride indicate the same Tafel line for that of platinum, since titanium is characterized by the evolution of oxygen. The curves obtained in the peculiar properties of its oxide film whose appearance 2N sulphuric acid solutions containing potassium is affected by various factors. Described below are its chloride, bromide or iodide indicate the Tafel line for polarization curves in the acid, neutral and alkaline the discharge of chloride, bromide or iodide ion at their solutions. characteristic discharge:potentials. Among neutral solutions, the fluoride solution alone shows the evolution In acid solutions; of oxygen owing to the discharge of hydroxyl ion, Fig. 2 shows the anodic polarization curves of tita- while potassium chloride and bromide solutions show nium in the acid solutions. the discharge of chloride and bromide ion at the dis- Fig. 2a shows spontaneouspotential vs. time curves charge potentials identical with that in the acid solu- obtained in the solutions. No change in potential was tions,. The discharge potential of iodide ion shows a observed either in 2N sulphuric acid solution or that 200 The Effect of Halide Ions on the AnodicPassivation of Titanium

containing potassium chloride or bromide. This seems discharge at 0.4～0.8 volt. In the solution eontaining to indicate that no change takes place in the oxide fluoride, the dissolutionof titanium continuesuntil the film formed in air, but the film is considered to be current density rises to the vicinity of 3mA/cm2, fol- under constant dissolution and reinforcement. In the lowing which titanium ceases to dissolve, polarization 2N sulphuric acid solution containing potassium flu- suddenly takes place, and the potential rises almost in ride, undissociated HF formed by KF+H2SO4→HF parallel with the potential axis. +K2SO4 dissolves titanium oxide chemieally in the Fig. 3 shows the results of an anodic polarization following way(1): experiment after the cathodic reduction of the titanium electrode surface at the current density of 1mA/cm2. The titanium metal also dissolves while generating In the 2N sulphuric acid solution and that contain- hydrogen. Accordingly, the potential-time curve be- ing potassium chloride, the polarization curves are comes less noble very rapidly. Iodide ion is unstable in steeper than in the case of polarization without catho- the acid solution and changes to I2 by the oxidation due dic reduction shown in Fig. 2. On the other hand, in to the dissolved oxygen. As a result, the potential- the 2N sulphuric acid solutions containing potassium time curve becomes noble by the oxiding power of bromide or iodide, no polarization is observed at a low free iodine. current density, and the polarization curve extends in The anodic polarization carried-out after 30 min of parallel with the current axis. At a definite current

Fig. 3 Anodic polarization curves for titanium after cathodic reduction (1mA/cm2, 30 min) in naturally aerated 2N-H2SO4 solution and that containing 0.1mol/l

potassium halide at 20℃.

Fig. 4 Anodic polarization curves for titanium in naturally aerated 0.1mol/l Dotassium halide solutions at 20℃.

immersion gives the polarization curves shown in Fig. density the oxide film begins to grow and polarization 2; the curves for the 2N sulphuric acid solutions are increses suddenly. When the discharge potential of shown in a previous report(2). The same polarization bromide or iodide ion has been reached, polarization is characteristics was also observed in the 2N sulphuric arrested by its discharge, and the polarization curve acid solution containing 0.1mol of potassium chloride. again extends-in parallel with the current axis. The Unlike the platinum electrode, the titanium electrode discharge of bromide or iodide ion continues up to a shows no discharge of chloride ion. In the 2N sulphuric higher current density than that for the specimens acid solution containing bromide, a short Tafel line of without cathodic reduction. bromide ion discharge appears at 1.2～1.4 volts. In the solution containing iodide, a Tafel line of iodide ion In neutral solutions: Fig. 4 shows anodic polarization curves of titanium (1) T. Smith and G.H. Hill: T. Electrochem. Soc., 105 (1958),117. (2) S. Morioka and A. Umezono: Trans. JIM, 5(1964),193 obtained in the neutral solutions containing potassium Susumu Mrioka and Akimi Umezono 201

halide salt 0.1Mol/l. surement of titanium in the 1N potassium hydroxide In the solutions of potassium fluoride or chloride, solution and that containing several potassium halides, polarization begins to increase with the increase of respectively. In this chart, the curves lacking in re- current density and is arrested by oxygen, evolution producihility are supplemented by marks indicating when the oxygen evolution potential has been reached. the probable,ranges,of existence. The potential of each It increases again when oxygen evolution ceases at a break in the curves is sufficiently reproducible. certain current density. There,appears no discharge Four distinct. regions are observed on the polarization of halide ions. There is a slight difference between curves. The potential arrest ranging from about 0.8 to the polarization characteristics observed in the solu- 1 volt represents the range corresponding to the evo- tions containing fluoride and chloride, because the lution of oxygen. This potential arrest is absent only solutions gave a slightly different pH value. In the on the polarization curve obtained in the solution con- potassium bromide solutions, the potential of titanium taining potassium iodide. Although iodide ion is ex-

Fig. 5 Anodic polarization curves for titanium after cathodic reduction (1mA/cm2, 30 min) in naturally aerated 0.1mol/l potassium halide solutions at 20℃.

Fig. 6 Anodic polarization curves for titanium in naturally aerated IN-KOH solution and that containing 0.1mol/l potassium halide at 20℃. oscilates rapidly between oxygen evolution potential pected to be able to discharge at a potential lower and,bromide ion discharge potential, resulting in the than that of hydroxyl ion even in an alkaline solution, floatation of white colloidal titanium hydroxide in, the no sign of iodide ion discharge can be seen on the po- solution. In the potassium iodide -solution, a Tafel line larization curve. It is probably the •simultaneous dis- indicating the iodide: ion, discharge is noted, and polar- charge with hydroxyl ion. ization increases to the oxygen evolution potential at When cathodic treatment is made!prior to anodic a high cur-rent density. When the current is momen- polarization, three regions are observed on the polari- tarily cutoff at this stage, the potential returns to the zation curve as shown in Fig. 7. Consequently, the iodide ion discharge potential and ,the oxygen evolu- third region ranging from about 1 to 2.0～2.2 volt tion potential can hardly be regained even if the cur- observed in Fig. 6 may represent the region correspon- rent density is raised. ding to the process in which the oxide film formed in Fig. 5 shows anodic polarization curves obtained air is gradually replaced by a non-conductive anodic after cathodic reduction of titanium suface in .neutral oxide film. solutions of potassium halide salt. Fig. 5 presents no Cathodic polarization: appreciable difference from Fig. 4. Fig. 8 shows the cathodic polarization curves. Tita- In alkaline solutions nium is attacked severely only in the acid solution con- Fig. 6 shows results of the anodic ,polarization mea- taining fluoride, but it is not attacked in other solu- 202 The Effect of Halide Ion on the Anodic Passivation of Titanium

tions,indicating the Tafel line for the hydrogenion dis- tinuous rise of potential up to above 3 volts, but the charge. It can be seen that the hydrogen overvoltage oxygen discharge begins gradually at 5～6 volts. The on titanium is considerably large as compared with discharge of chloride ion seems to occur even a at that on platinum (Fig. 1). The titanium surface where higher potential. In this connection, Hall & Hacker- the evolution of hydrogen has taken place is hardened man(3) state that pitting corroion beging to take place

Fig. 7 Anodic polarization curves for titanium after cathodic reduction (1mA/cm2, 30 min) in naturally aerated IN-KOH solution and that containing 0.1mol/l potassium halide at 20℃.

Fig. 8 Cathodic polarization curves for titanium in naturally aerated solutions at 20℃. and appears blackish suggesting the formation of at 10 volts (vs. S.C.E.), while Otsuka(4) reports the hydrides. occurrence of pitting corrosion at 12 volts (applied voltage). IV. Consideration This fact proves stronger affinity of titanium with oxygen than with chlorine, resulting in prior discharge The discharge potentials of. various anions on the of hydroxyl or sulphate ion. It appears that titanium platinum anode is nobler in the following order as has excellent resistance to corrosion by sea water and shown in Fig. 1: is very strong against pitting corrosion by solutions containing chloride ion, On the other hand, the

Note: [ ] indicates the absence of discharge passivated stainless steel anode stands with the platinum anode in the halide ion discharge potential(5). under natural conditions. Stainless steel is also susceptible to pitting corrosion In case of the titanium anode, the order of ion dis- in sea water. From these facts, it can be understood charge potential will be that there is some relationship between the strong resistance of titanium against pitting corrosion and the difficulty of creating chloride ion discharge on as shown in Figs.2～7. In the anodic polarization of titanium, the oxygen the titaninm anode. discharge potential is not so much different from that The mechanism of halide ions causing pitting corrosion through the inhibition of the passivation of me- in the case of platinum, but.the discharge overvoltage of halide ion is large. A remarkable difference from tal may be explained in the following way. It is an erring view that chloride ion, for instance, is so small platinum is found ,in the absence of chloride ion discharge from the acid and neutral solutions, and also that it penetrates through defects in the oxide film to in the absence of iodide ion discharge (or possibly si- the metal surface and activates the metal. Instead, multaneous discharge with oxygen) from the alkaline electrons liberated by the discharge of anions on the solutions, It was previously reported(2) on the anodic oxide film surface should penetrate the film and reach polarization in acid solutions that when the oxide film (3) C. D. Hall and N. Hackerman: J. Phys. Chem., 57(1953),262. formed in air becomes non-conductive, the evolution (4) R. Otsuka: J. Electrochem. Soc. Japan, 26 (1958),619. (5) S. Morioka and K. Sakiyama: J. Japan Inst. Metals., 18 of oxygen ceases at 1.7～1.9 volts and shows a con- (1954),643,647, J. Electrochem. Soc. Japan., 25 (1957),191. Susumu Morioka and Akimi Umezono 203 the metal; the routes of penetration by the electrons become channels for the penetration of metal ions to V. Summary the surfaces). If it is the discharge of oxygen ions, The effect of halide ions (0.1mol/l.) on the passi- metal ions having reached the surface are combined vation of titanium in acid (2N sulphuric acid), neutral with oxygen ions to form a film or fill the existing and alkaline (1N potassium hydroxide) solutions has film defects by M++＋O-→MO. The passivity is there- been studied. The results are as follows: fore maintained in this case. In the case of halide ion (1) In the presence of fluoride, a severe attack of discharge, however, they produce soluble salts and titanium occurs in the acid solution, as undissociated thereby the titanium ions become hydroxide by hydrol- HF dissolves chemicallytitanium oxide film. Fluolide ysis leaving the film defects unremedied. In the neu- ion does not precipitate through the discharge from tral bromide solution, for instance, bromide and hydr- the aqueoussolution, and therefore it does not prevent oxyl ion discharge potentials are not widely separated, passivation after a certain current density has been and therefore the oxide film is incessantly repaired reached. No attack of titanium takes place in the ne- and destructed by their discharge after the current utral or alkaline solution. density has reached a certain level. This results in (2) Chloride ion does not prevent the passivation of the fluctuation of the potentials shown in Figs. 4 and titanium in any solution as its discharge is prevented 5 and pitting corrosion. In the case of iodide ion dis- by a very high discharge overvoltage on titanium. charge, the discharge potential is very low and the cur- (3) Bromideion discharges from the acid and neu- rent is distributed evenly over the film surface from di- tral solutions, causing pitting corrosion in the latter verse points of discharge, causing no pitting corrosion. solution. The discharge overvoltage is higher than in The most common halide ion which has higher dis- the case of dischargeon a platinum electrode. chargeability than hydroxyl is chloride. On the ti- (4) Iodide ion also- discharges from the acid and tanium surface, however, chloride is hardly discharge- neutral solutions at a high discharge overvoltage, but able, and therefore chloride ions cause no pitting it does not discharge from the alkaline solutions (or corrosion under natural conditions. This is why tita- possibly simultaneous dischage with hydroxyl ion). nium is remarkably high in resitance against pitting Even when it discharges, it does not cause pitting corrosion. Natural corrosion seems to entail pitting corrosion. corrosion by the absence of a compact anodic oxide (5) A study of the mechanism of halide ions which film, but this is a wrong presumption. In fact, pit- prevents the passivation of various metals leads to the ting corrosion scarcely occurs in such a case, because clarification of why titanium has high resistivity to the anode potential of the local cell can hardly reach pitting corrosion. the level of halide discharge potential owing to the (6) According to the result of measurement of cath- corrosion current. odic polarization, hydrogen overvotlage on titanium is considerably high. (6) H.E.Haring: J. Electrochem. Soc., 99 (1952), 30.