Indian Journal of Chemical Technology Vol. 4, March 1997, pp. 101-109 Effect of some anions on corrosion behaviour of 1060,1100 and 5052 aluminium alloys in orthophosphoric acid R S Dubey", R Sagar Dubey, Y D Upadhya &. S N Upadhyay Department of Chemical Engineering and Technology, Institute of Technology, Banaras Hindu University, Varanasi 221 005,'India Received 26 August 1996; accepted 26 November 1996 The effect of chromate, citrate, thiocyanate, tartarate, oxalate and ferrocyanide ions on the corro- sion behaviour of 1060, 1100 and 5052 aluminium alloys in 1% orthophosphoric acid has been studied by weight loss technique at 30 and 40 ± OSC, whereas corrosion behaviour of 1100 aluminium alloy in the presence of above anions was studied by potentiostatic polarisation technique. It was observed that these anions play a decisive role in determining the ease with which the above alloys undergo dissolu- tion in orthophosphoric acid. Chromates, oxalates and ferrocyanides worked as corrosion inhibitors, whereas citrates, tartarates and thiocyanates accelerated the corrosion reaction at all the concentrations (0.01-0.2%) investigated. The stability of the surface film formed in the presence of these anions were closely related to the stability constant of metal-chelate complexes. The presence of anions in aqueous solutions plays of the interesting examples of corrosion inhibitors. an important role in determining the corrosion be- These are ions or molecules with the two or more haviour of aluminium and its alloys. An anion may atoms containing an unshared pair of electrons. act either as a corrosion inhibitor or as an acceler- They donate the lone pair of electrons to the ca- ator. To study the inhibition effect on corrosion of tion to form a stable five or six membered ring, as aluminium alloys by these aggressive anions, the a result of which metal atom is held in a stable important points are the steps by which these an- configuration. The complexes neither exhibits the ions act on the aluminium. surface'. These steps properties of the metal atom nor those of the che- involve the adsorption of anions on the aluminium lating agent. These chelating agents can react 'with oxide surface, complexing of aluminium cations in aluminium cations in oxide film". The stability of oxide lattice with anions present in the electrolyte, th complex formed depends upon the nature of thinning of protective oxide film by the dissolution the chelating agents. of soluble corrosion products and direct reaction The objective of the present work was to inves- of aluminium with the electrolyte at sufficiently tigate the effect of some chelating agents on the thinned sites. dissolution behaviour of aluminium oxide film In the case of orthophosphoric acid, the anions formed on the aluminium surface. To fulfil this ob- may compete for adsorption sites and retard the jective, some organic and inorganic chelating formation of soluble aluminium hydrogen phosph- agents such as potassium sulphate, potassium thi- ate or they can compete with POl- to prevent the ocyanate, potassium persulphate, potassium formation of soluble phosphate species. But in the chromate, potassium citrate, potassium tartarate, latter case, it must be recognised that if an inor- potassium oxalate and potassium ferrocyanide ganic anion forms a stable soluble complex ion were used as corrosion inhibitors. with the aluminium cation, the dissolution will proceed just as it could otherwise proceed with Experimental Procedure the formation of aluminium hydrogen phosphate. Orthophosphoric acid, potassium salts of the In the case of aluminium corrosion, the electro- above mentioned anions (AR grade) and triple lyte containing the inhibitor will be in direct con- distilled water were used throughout the experi- tact with its oxide film. Therefore, the possible. ments. All the weight loss and potentiostatic po- corrosioin mechanism must include the interaction larisation experiments in the present work were of the anion (or inhibitor) with aluminium cation carried out in 1% orthophosphoric acid solution. in the oxide film. Chelating agents are among one The potentiostat used in this experiment was •Author to whom correspondence should be addressed Wenking model POS-73. Polarisation experiments 102 INDIAN J. CHEM. TECHNOL., MARCH 1997 Table I-Compositions of aluminium alloys (wt%) eration of corrosion by the above mentioned an- ions can be explained in view of their interaction Alloy Si Fe Mn Mg Cu AI 1060 0.12 0.02 0.04 Remainder with aluminium oxide film. The oxide film formed 1100 0.13 0.32 0.07 0.02 0.01 Remainder adjacent to the aluminium surface is a thin com- 5052 0.17 0.04 2.30 Remainder pact barrier film"!', whereas a thicker more permeable layer is also present in the bulk of the were carried out in a 250 mL pyrex glass cell con- film 1• In the presence of chromates, oxalates and taining an auxiliary platinum electrode and a satu- ferrocyanides the corrosion resistance may be at- rated calomel reference electrode. A flag shaped tributed to the incorporation of these anions to piece of aluminium with the stem coated with lac- the continuous oxide layer next to the metal. The quer to have a working area of 2 ern? was used as magnitude of the corrosion rate is expected to de- the working electrode. The reference electrode pend on the solubility and composition of the bulk was connected with the help of a Luggin capillary film". which was placed close to the working electrode. Effect of potassium chromate-The inhibition ef- The distance between the capillary tip and the ficiency of potassium chromate towards 1060, working electrode was kept constant throughout 1100 and 5052 aluminium alloys is given in Tables the experiment. Open circuit potential (OCP) was 2-4. It is clear from the results incorporated in recorded after 30 min of exposure of aluminium these tables that potassium chromate acts as a alloy samples to the test solution. During poten- good corrosion inhibitor for these aluminium al- tiostatic polarisation experiments, the potential loys in 1% orthophosphoric acid solution. In the was changed in an instalment of 50 mV after re- presence of of chromate ions the open curcuit cording the steady state current density. The com- potential (OCP), shifted in noble direction. positions of the aluminium alloys used in this Anodic and cathodic polarization curves for study are given in Table 1. All the samples were 1100 aluminium alloy in 1% orthophosphoric acid polished with emery paper of grades 1/0 to 4/0. at various concentrations of chromate are shown The specimens were then cleaned and degreased in Fig. 1, whereas different electrochemical par- in acetone. Experiments were carried out in elec- ameters are listed in Table 5. It is obvious from tronically regulated air thermostat maintained at this figure that anodic polarization curves shifted 30 ± OSC and 40 ± OSc. towards the lower current density region with in- crease in concentration of potassium chromate. Results and Discussion However, the nature of the curves remains almost Weight loss studies-Corrosion data obtained the same. It is also c1ear that anodic polarisation from immersion tests for 1060, 1100, and 5052 al- curves are slightly polarised in the presence of po- uminium alloys at 30°C are given in Tables 2-4. tassium chromate. Both anodic and cathodic parts Influence of these anions on above aluminium al- of the polarisation curves shift in the same direc- loys have been determined at various concentr- tion with increase in the concentration of chrom- ations of the inhibitors ranging from 0.01 to 0.2%. ate ions. It is clear from these tables that potassium chrom- It has been reported by Kravchenko et al.13, ate, potassium oxalate and potassium ferrocyanide that chromate is more effective as an inhibitor worked as corrosion inhibitors, whereas potassium than molybdate, permanganate, ferrocyanide and thiocyanate, potassium citrate and potassium tar- nitrate. The mechanism of the inhibition of alumi- tarate accelerated the corrosion reaction at all the nium corrosion by chromate has been studied by concentrations investigated. many workers'vP, It has been found that chrom- The. corrosion behaviour of 1060, 1100 and ate acts as an oxidizer in the inhibition of alumini- 5052 aluminium alloys in orthophosphoric acid at um corrosion. The Cr203 gets incorporated into different concentrations and the mechanism of dis- the passive nlm of aluminium. This is responsible solution of passive film on aluminium alloy surface for a good inhibition of aluminium by chromates. have been reported by Dubey? in recent studies. Effect of potassium thiocyanate-Potassium thi- The corrosion resistance of UOO aluminium alloy ocyanate is usually regarded as an activator of was found to be highest followed by 1060 and corrosion of iron" by activating the dissolution of 5052 aluminium alloys. Therefore, 1100 alumini- passive film. Thiocyanate ion is known to be a um was selected to investigating the effect of var- good complexing agent for iron ion'".'{he anodic ious aggressive anions in the presence of ortho- polarization curves of 1100 aluminium alloy 1% phosphoric acid by electrochemical polarisation .orthophosphoric acid and at various concentr- technique. The mechanism of inhibition and accel- ations of thiocyanate ions are given in Fig. 2. Shift DUBEY et al: CORROSION BEHAVIOUR OF SOME AWMINIUM AUDYS 103 Table 2-Percentage inhibition efJiciency of 1060 aluminum Table 3-Percentage inhibition of 1100 aluminium alloy in the alloy in the presence