Effect of Some Anions on Corrosion Behaviour of 1060,1100 and 5052 Aluminium Alloys in Orthophosphoric Acid
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 of different concentrations of anions at presence of different concentrations of anions at 30 and 40 30 and 40 ± OSC ±OSC Exposure period = 48 h Exposure period - 48 h Inhibitor and 30·C 4O·C Inhibitor and 30·C 40·C percentage percentage concentration Wt.loss Percentage Percentage Wt.loss concentration Wt.loss Percentage Wt.loss Percentage mg efficiency mg efficiency mg efficiency mg efficiency Plain solution 125 249 E E Potassium Plain solution 147 6 292 chromate Potassium om 100 21 186 25 chromate 0.02 79 38 144 42 om 121 18 216 26 0.05 57 55 107 57 0.02 105 28 193 34 0.10 48 62 82 67 0.05 101 31 180 38 0.15 41 68 70 72 0.10 80 45 129 56 0.20 31 76 47 81 '0.15 64 56 105 64 Potassium 0.20 51 65 85 71 Thiocyanate Potassium 0.01 134 -5 266 -7 thiocyanate 0.02 138 -9 278 -12 om 160 -9 327 -12 0.05 142 -12 286 -15 0.02 169 -15 344 -18 0.10 ISO -18 301 -21 0.05 173 -18 353 -21 0.15 153 -21 317 -25 0.10 185 -26 385 -32 0.20 160 -26 328 -32 0.15 194 -32 400 -37 Potassium 0.20 202 -38 420 -44 citrate Potassium citrate om 135 -7 269 -8 164 -12 338 -16 0.02 144 -13 291 -17 0.01 0.02 173 -18 362 - 24 0.05 ISO . -18 296 -19 185 -22 376 -29 0.10 154 -21 311 -25 0.05 0.10 -28 402 -38 0.15 166 -31 336 -35 188 199 -36 411 -41 0.20 171 -35 343.6 -38 0.15 Potassium 0.20 208 -42 429 -47 tartarate Potassium tartarate 0.01 132 -4 261 -5 0.01 154 -5 315 -8 0.02 135 -7 274 -9 0.02 187 -7 321 -10 0.05 138 -~ 281 -13 0.05 158 -8 327 -12 0.10 142 -12 296 -19 0.10 166 -13 338 -16 0.15 147 -16 301 -19 0.15 173 -80 356 -22 0.20 149 -18 311 -25 0.20 177 -21 374 -28 Potassium Potassium oxalate oxalate 0.01 104 18 189 24 0.01 169 15 231 21 0.02 95 25 172 31 0.02 177 21 213 27 0.05 79 38 137 45 0.05 202 38 177 41 0.10 62 51 102 59 0.10 217 48 140 52 0.15 46 63 82 67 0.15 229 56 123 51! 0.20 41 68 69 72 0.20 242 65 98 71 Potassium Potassium ferrocyarude ferrocyanide om 121 5 231 7 om 154 5 269 8 0.02 116 8 221 l\ 0.02 163 II 248 15 0.05 104 18 196 21 0.05 167 14 242 17 0.10 Hhl 21 184 26 0.10 17.1 18 .~22 24 0.15 ':I!. 27 171 11 O.I~ 1M2 24 :'10 28 ,!.{'; u -.J"~ 35 157 J' 02( }'H 32 17I 41 .. -'---"-'-' -.- --"'--'-'-~-.'..- _ ....-.--.. -. _._--_._----_ ..-._------_.-.- 104 INDIAN J. CHEM. TECHNOL., MARCH 1997
Table 4-Percentage inhibition efficiency of 5052 aluminium Table 5-Electrochemical parameters of corrosion of 1100 alloy in the presence of different concentrations of anions at aluminium alloy in 1% H3P04 with different anions
30 and 40 ±OSC Inhibitor Ecorr Icorr2 IE Exposure period ~ 48 h (0.2%), w/v mV/SCE mAlcm2 % 1% orthophosphoric acid - 720 0.7 Inhibitor 30°C 40°C percentage Pot. chromate - 710 0.13 28.57 Wt. loss Percentage Wt.loss Percentage -21.43 concentration Pot. thiocyanate -740 0.58 mg efficiency mg efficiency Pot. citrate -750 0.95 -35.7 E Pot. tartarate -755 0.92 -31.4 Plain solution 160 370 Pot. oxalate - 720 0.25 64.3 Potassium Pot. ferrocyanide -710 0.20 28.6 chromate om 137.6 4 292.3 21 of the anodic polarisation curves towards higher 0.02 126.4 21 266.4 28 current density region with increase in concentra- 0.05 116.8 27 151.6 32 tion of potassium. thiocyanate suggest the activa- 0.10 92.8 42 196.1 47 tion of corrosion reaction in the presence of thioc- 0.15 78.4 51 162.8 56 yanate ions The activating behaviour of thiocyan- 0.20 67.2 58 155.4 65 ate ions may be attributed to the strong ligand Potassium field around the metal ions. Complexing of thiocy- thiocyanate anate ion with AP + resulted in the extraction of om 177.6 -11 432 -17 aluminium ion from the Al203 film. The active 0.02 188.8 -18 466 -26 dissolution of aluminium from the surface oxide 0.05 198.4 -24 496 -34 film in presence of thiocyanate ion is clear from 0.10 209.6 -31 537 -45 the weight loss data incorporated in Tables 2-4. 0.15 216 -35 562 -52 The complex compounds formed during the reac- 0.20 227 -42 596 -61 tion with aluminium are very stable in solution. Potassium citrate Effect of potassium citrate and potassium tartar- om 184 -15 432.9 -17 ate-Potassium citrate and potassium tartarate are 0.02 193.6 -21 462.5 -25 salts of hydroxy carboxylate acids. The inhibition 0.05 206.4 -29 492.1 -33 efficiencies of these inhibitors at various concentr- 0.10 216.0 -35 514.3 -39 ations are given in Tables 2-4. Anodic polarization 0.15 225.6 -41 540.2 -46 curves of 1100 aluminium alloy in 1% orthophos- 0.20 236.8 -48 569.8 -54 phoric acid with different concentrations of pota- Potassium tartarate ssium citrate and pot. tartarate are plotted in 0.01 171.2 -7 403.3 -9 Figs 3 and 4, respectively. Both potassium citrate 0.Q2 179.2 -12 425.5 -15 and potassium tartarate are chelating agents. It is 0.05 142.4 -14 436.6 -18 clear from the anodic polarization curves that 0.10 148.0 -15 447.7 -21 these ions accelerate the corrosion of aluminium 0.15 190.4 -19 463.0 -25 at all the concentrations investigated. Although 0.20 198.4 -24 488.0 -32 there is only a slight shift in OCP towards nega- Potassium oxalate tive direction, the loss in weight ,pbserved in their 0.01 140.8 12 311 16 presence is of considerable extent. Bryan" carried 0.Q2 118.4 26 252 32 out a series of investigations on the corrosion of 0.05 105.6 34 218.3 41 aluminium alloys in citric and tartaric acids. He 0.10 89.6 44 196 47 found that the solubility of AI(OH)3 in these acids 0.15 76.8 52 155 58 was more than double of that needed to combine 0.20 67.2 58 129.5 65 with the hydrogen of the - COOH groups. It was Potassium further observed that the rate of corrosion of alu- ferrocyanide minium increased by decreasing the concentration 0.01 153.6 4 344 7 of citric acid. This was unaffected by the addition 0.Q2 146.6 9 325 12 of sodium citrate, but the presence of NaCI in- 0.05 140 12 307 17 creased the rate of corrosion and evolution of hy- 0.10 120 25 251.6 32 drogen with increasing acid concentration. 0.20 115.2 28 229.4 38 Katoh 16 reported that pure aluminium did not corrode in 1M citric acid solution because of the DUBEY- et al: CORROSION BEHAVIOUR OF SOME AWMINIUM AlLOYS 105
2~1.-~------~ __ ~~-n--~~~------_
1500
V 1v. H)P04 ptain o 0'01'/, Pot.chromate + 1·0'l.H3~ t! 1000 A 0'02'1,
-1000
-IS00 L,.----'------'--'--'---'-..LJ.. ..•..~.L,o2.----....L..-----''--'--'---'-..LJ...L~.L,CJ3.----.;a...--~~-'-·..u,04 , O Current density, ]!A /,,,,2 Fig. I-Anodic and cathodic polarization curves for 1100 alu- minium alloy in 1.0% H)PO. + potassium chromate
2000r------~~~~~~
1500 ~ '·O'l.H)POt, Plain o 0'01'1. Pot· thiocyanate + '·O'l.H3PO, I::i. 0-02-'. " ~ '000 ~ D 0-05'1. > • 0'10'1, E • 0·1S'I. .2 SOO • 0·20'1, c•. ~ 0. o ~ ~ u .!
W-500~-~~
-'OOO~
Fig. 2-Anodic and cathodic polarization curves for 1100 alu- minium alloy in 1.0% H)PO. + potassium thiocyanate
protective oxide film. Addition of fluoride ions, where however, increased the corrosion rate. The stabil- o OH 0 ity constants of citrate ions formed due to com- II I II plexation are given below"; L= -C-C-CH2-C-CH2-C-0- I Reaction Temp. Ionic Log Stab. COO DC Strength Constant o 0- 0 Al3+ + H)L= AIL + 3H+ 33 (0.25 NaCIO.) 4.7 II I II Al(HiLt + H" = AlL 33 (0.25 NaCI04) 3.5 HiL= -O-C-CH2-C-CH2-C-O- Al(OHXH - IL)- - + H" I =AI(HiL) 33 (0.15 NaCI04) 6.8 COO- 106 INDIAN J. CHEM. TECHNOL., MARCH 1997
2~r------9~o.~~-,
<;J 1'0'1. H3PO, Plain o 0'01 'I. Pol.larla(ale + 1·0'l.H3PO, ••• ~ 0-02'1. ;;:1000 o 0-0!>'1. > • 0-10'1. e • 01!>'1. E 500 • 0-20'1. c • ~0. .•., o e u .! •••-500
Fig. 3-Anodic and cathodic polarization curves for 1100 alu- minium alloy in l.()'}'oHjPO. + potassium citrate
2oo0r------~~_C~hr __
1500 <;J 1'1. H) P04 Plain o 0'01'1. Pol_c itrat. .• 1-0 ./. H3 PO, ~ 0'02'1. " 0-05'/. It u••• 1000 o ~ • 0-1',. ,. > • 0'15'1. E • 0-20'1. a 500 - C. & 0 '0. e u ;;,-500
-1~
Fig. 4-Anodic and cathodic polarization curves for 1100 alu- minium alloy in 1.0% H)PO. + potassium tartarate
It is obvious from the above data that the soluble AP + + Cit" - - AICit- chelates accelerate the dissolution of aluminium. Some of the important equations suggested by Ka- toh 16 in his discussion on corrosion of aluminium ... (2) in citrate solutions are AI- AP+ + 3e- AlCit+ +OH- -(OH) AlCitl-,
2 0.059 3+ K _[(OH) AICit -1_ 6_6 E= -1.66+- -log(AI ) ... (1) ... (3) 3 2 - [AICit IOHl -10 DUBEY et al: CORROSION BEHAVIOUR OF SOME AWMINIUM AlLOYS 107
2~r------~~~--~~------'
1500 . <:7 1'0'1. HJP04 Plain 0 O-ol'l.Pot·oxalale • I'O%H)PO, L;j 6 (t02% .. " u 1000 a 0-05'1. ~ • 0-10% > •• .0·15'/, e 0"20'/, 500 • .2 ~ • 0Q. 0 -e·e ... W• -500
-1000
Fig. 5-Anodic and cathodic polarization curves for 1100 alu- minium alloy in 1.0% H)PO. + potassium oxalate
2000r------~~~~~~------_,
1500
<:7 I'l.HlPCl{, Plain - o O·Ol'!. ~t.f.rrocyanitH + I·O'l.HlP04 ~'" 1000 6 0'02'1. •• o O'OS% >e • 0'1% •• 0'15% ~ 500 • 0'20"10 ~ z o Q. • o ""e ... ~ -500
Fig. 6-Anodic and cathodic polarization curves for 1100 alu-
minium alloy in 1.0% H)P04 + potassium ferrocyanide
The acceleration of corrosion of aluminium in O:SM and below, the soluble chelate complexes do citrate solution can be explained with the help of accelerate the attack even' in absence of phosphate Eqs (1)-(3) above. Tartarate ions also play a simi- ions". lar role in accelerating the corrosion of aluminium. Effect of potassium oxalate and potassium ferro- It has been reported by several workers 18 that cyanide-The anodic and cathodic polarization tartaric acid is corrosive to aluminium. Lorcking'? curves of 1100 aluminium alloy in the presence of reported that 0.1 N sodium tartarate solution has potassium oxalate and potassium ferrocyanide are no inhibitive' effect on corrosion of aluminium. It shown in Figs 4 and 5 respectively. It is clear appears that in concentrated solutions, the equili- from these figures that in presence of these anions, brium is such that the stable complexes do not anodic polarization curves shift towards the lower accelerate corrosion. However, at concentration of current density region indicating that corrosion 108 INDIAN J. CHEM. TECHNOL., MARCH 1997
..... I ..... : I operating across the oxide film opposing transport AI :... I . ....I -: .. of either the aluminium ions towards the electro- ...::::. I " : .: :: ~:. lyte or oxygen containing anions towards the me- Oxi ••• I .: Solution .::.....I tal. The field stimulates only the transport of the AI .. ;:"-:: ~ : electrons causing reduction of H+ ions and evolu- F.1d o~ni"!ll tra~ I .;:' ..:; I AI tion of hydrogen at the oxide surface. A reversal ,#,S of the direction of the fields takes place by imposing a suitable voltage from an external :::".::.:':'.:.: Pllalj)/lorlc acid source to enable the ionization of aluminium and .... ::...... :.: further build the oxide layer (Fig. 7a). An entirely different situation may be created by adsorption of negatively charged species at the oxide/electrolyte interface as shown in Fig, 7a. If the concentration of negative charge becomes (e ) Ib' higher on the side of the oxide layer than on the Fig. 7-Schematic representation of the AI-solution interface metal side, the direction of the field is reversed
withgalvanic potential distribution in H3P04 (a) at the open and it assists dissolution, i.e., transport of the alu- circuit potential, (b) during the anodization minium ion, towards the electrolyte and the oxy- gen ions in the reverse direction. The mechanism current decreases considerably in the presence of also holds true for a lower valency state of AI these anions. The same result is also observed (e.g., AI +) which could pass through the film and from weight loss data given in Tables 2-4. Thus reduce water at the oxide/ electrolyte interface. oxalate ions behave as a corrosion inhibitor for This constitutes an additional mechanism for hy- this metal-electrolyte combination. It has been re- drogen evolution. ported in literature-P-" that oxalic acid and tartaric The proposed model also allows for differences acid are corrosive to aluminium and the rate of in the effects of various anions. It is sufficient to corrosion is a function of pH. But in the presence assume different adsorbability (energies of adsorp- of potassium oxalate in H3P04 solutions, a thin tion) between the anions and oxides. That would film is formed which is visible even to naked eye. imply different potentials at which significant ad- Oxalate ions shift the OCP to the noble direction. sorption takes place and hence sudden activation Consequently, a decrease in corrosion current is of anodic dissolution. observed at various concentrations of the pota- ssium oxalate. The protective nature of potassium Conclusion oxalate may be due to a precipitation type of inhi- Chromate, oxalate and ferrocyanides were bition. The possible reaction of AP + with oxalate found to inhibit the corrosion of 1060, 1100 and are: 5052 aluminium alloys, whereas citrate, tartarate and thiocyanate accelerated the corrosion reaction. Reaction Log. Stability Constant Inhibition effeciency increases with the inhibitor (Ionic Strength 1M) concentration and temperature of the test solution. AP+ + C20~- -+ AIC20: 6.06 1060 aluminium alloy possess better corrosion re- sistance than 1100 and 5052 aluminium in the AP+ + 2C20~- -+ AI(C204)i 11.09 presence of these anions. 15.12 Acknowledgement From the above results it is clear that anions The authors thank to M/ s HINDALCO, play a decisive role in determining the ease with Renukoot, Mirzapur for providing the samples of which aluminium undergoes dissolution upon ap- aluminium alloys. One of the author (RSD) is plication of some anodic current density. thankful to Banaras Hindu University for financial For such a system, a model based on anion support. adsorption can be proposed which explains the activating role of anions. In the absence of any References adsorption, the cross section of the interface can 1 Nguyen T H & FoleyR T, J Electrochem Soc, 126 (1979) be schematically represented as shown in fig. 7. 1855. 2 Takahashi H, Fujimoto K & Nagagama M J, J Electrochem At OCP (Fig. 7), the surface of the metal is nega- Soc, 135 (1988) 1343. tively charged and hence an electric field is 3 Dubey RS, Bull Electrochem; 12 (1996) 467. DUBEY et al.: CORROSION BEHAVIOUR OF SOME ALUMINIUM AlLOYS 109
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