Pitting corrosion of aluminum
Z.Szklarska-Smialowska
鹤嘴锤
Fontana Corrosion Center,The Ohio State University,Columbus,OH,43210,USA
Received 14August 1998;accepted 4September 1998半夏
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This review describes the experiments performed during the last few decades which enhance knowledge of the pitting of aluminum.Speci®cally,metastable and stable pits,pit chemistry and the e ect of intermetallics on pitting are discussed.The properties of metastable alloys and inhibition of Al are also discussed.#1999Published by Elsevier Science Ltd.All rights reserved.
Keywords:Pitting;Aluminium;Passivity;Inhibitors;Intermetalics
1.Introduction
Most of this article reviews some aspect of the passivity and pitting of Al and Al-alloys.It has been sugg
ested that anodic oxide ®lms might control the corrosion resistance of the base metal.It is known that di erent factors in¯uence the pitting of other metals and alloys the same way as that of aluminum and aluminum alloys and that the existing hypothesis of pitting mechanisms in halogen environments apply to other metals and alloys as well.Recently,the interest in localized corrosion and inhibition of Al-alloys has been revived intensely because of aging of Al-alloys which are used extensively in the airspace industry ad because of increasing use of Al-alloys in the automotive industry.Four stages of pitting corrosion can be distinguished:(1)processes occurring on the passive ®lm,at the boundary of the passive ®lm and the solution;(2)processes occurring within the passive ®lm,when no visible microscopic changes occur in a ®lm;(3)formation of so-called metastable pits which initiate and grow for a short period of time below the critical pitting potential and then repassivate (this is an 0010-938X/99/$-see front matter #1999Published by Elsevier Science Ltd.All rights reserved.PII:S 0010-938X (99)00012-8
Corrosion Science 41(1999)
1743±1767
intermediate step in pitting);and (4)stable pit growth,above a certain potential termed the critical pitting potential.A vast number of papers have been published through the years on the growth of stable pits.Metastable pits were ®rst described qualitatively about 30years ago,but quantitative studies were not reported until the 1980s [1].Concerning the ®rst two stagesÐthe processes which lead to the breakdown of the ®lm,hence the interaction of Cl Àwith an oxide ®lmÐlittle is known.These stages are certainly dependent upon the composition and structure of the oxide ®lm.The structural characteristics of the oxide depend on the material composition,the presence and distribution of micro-defects (vacancies,voids,etc.)as well as macro-defects (inclusions,second phas
绝对值角度编码器e particles its size and shape),crystal structure and the degree of noncrystalinity of the oxide.Its also depends on the electrolyte composition,potential and temperature.In neutral (roughly between pH 4±9)noncomplexing solutions,the oxide ®lm on aluminum has very low solubility.Its electronic conductivity is also very low;hence the redox reactions are blocked.However,a small current is measured during metal polarization as a result of the presence of defects in the oxide ®lm.The air-bourne oxide on the Al was observed to be amorphous,whereas,the structure of the oxide obtained by thermal oxidation of Al was strongly dependent on the temperature.Anodic ®lms that were grown in a borate and tartaric acid solution on the Al are thin,dense,coherent and amorphous,whereas ®lms grown in sulfuric and phosphoric acids contain layers which are thick,porous and crystalline.Hence,the structure of the oxide ®lm on aluminum can be di erent resulting in di erent chemical and physical properties.Vijh [2]has that the corrosion potentials of Cu,Ni,Sn,Pb,Fe,Al and Zn in chloride solutions can be related to the estimated band gap values of the corrosion reaction ®lms most likely to be formed.Highly insulating ®lms tend to be associated with more cathodic potentials;a fact which has been qualitatively explained in terms of electron-hole participation in the partial reactions proceeding on the ®lm covered electrode during open-circuit corrosion.The oxide ®lms grown in aqueous environments on the majority of metals and alloys display semiconductive properties.There are many papers studying the semiconductive properties of iron and steels but only few papers exist that deal with electronic properties of aluminum passive ®lm.The semiconductive properties of passive ®lms were observed to vary with environmental and material variables.For example,depending upon the heat treatment,the passive ®lms on stainless steels exhibited either p -or n -type of defects.At higher electrolyte temperatures,the oxide ®lms on 304stainless steel behave predominantly as an n -type conductor [3].When higher anodic voltages are applied,the space charge region can be inverted to a p -type for an n -type conductor,and to an n -type for a p -type semiconductor [4].The number of pits was greater when the ®lms were of the n -type rather than the p -type [5].Several researchers have intended to correlate the intrinsic properties of oxide ®lms with pitting susceptibility,studying the semiconductive properties of oxides using AC-impedance and photoelectrochemical techniques.The photo-e ect easily
Z.Szklarska-Smialowska /Corrosion Science 41(1999)1743±17671744
Z.Szklarska-Smialowska/Corrosion Science41(1999)1743±17671745 can be related to the semiconducting nature of the passive®lm.The concentration of carriers near the surface can be increased by illumination.Lenhard et al.[6] found a signi®cant decrease in the number of pits and an increase in the breakdown potential,when the nickel oxide(p-type)was illuminated.At the same time the pitting potential moved to a more positive potential.The same phenomenon was found on steel[7].
Morach et al.[8]found that materials with a lower number of localized states have better resistance to localized corrosion. Bulk Al2O3is an insulator with a band gap8±9eV[9]but the passive®lm on the Al exhibits a band gap H3eV[10].Menezes et al.[10]measured the photocurrent as a function of potential for pure Al,and AA7075and AA3003 alloys in chloride,molybdate and sulfate solutions.They found that the¯at band potential is dependent upon the kind of electrolyte present and shifts to a more positive value with increasing susceptibility to pitting,which is probably associated with a defect structure of the ,deviation from stoichiometry). According to the authors,the diminished photoresponse at positive potentials suggests that the defects(O vacancies or Al interstitials)migrate toward the interface and oxidize.
Kobotiatis et al.[11]studied the electronic properties of a passive layer grown anodically on Al70775in chromate and oxalate solutions using electrochemical impedance spectroscopy.The Mott±Schottky plots were obtained from the measurements of capacitance at di erent potentials.Flat band potentials and the density of the energy state were calculated from these plots.Di erences were found in semiconducting properties of the passive layer formed in chromate and oxalate solutions.These di erences were related to the inhibition properties of chromate and oxalate to pitting.The oxide developed in the presence of chromate (good inhibitor)exhibits a less-noble¯at-band potential and a
lower average density of state.No e ect of the electric potential was found on the¯at band potential and the density of the energy states.Hence,in both of the above papers, the¯at band potential was less noble in conditions when the susceptibility of Al to pitting is low as stated in the literature.
The e ect of the electronic structure on pitting can be explained by a point defect model[12].This model assumes that the chloride ions incorporate to the passive®lm by occupying anion vacancies.This results in a decrease of anion vacancies and increases the cation vacancies.When the cation vacancies start to pile up at the metal interface,a breakdown of the®lms occurs.
2.Interaction of chloride anions with passive®lm
In an e ort to establish the interaction of chloride ions with a passive®, if chloride ions are adsorbed on the Al-oxide®lm and/or are incorporated in the oxide®lm.Di erent analytical techniques have been used.
Analytical techniques such as autoradiography[13]SIMS[14]and X-ray photoelectron spectroscopy[15]revealed adsorption of ClÀions on passive®lms of Al at the open circuit potential as well as at applied anodic potentials.The
metal surface was found to be inhomogeneous,consisting of a number of defects,and the adsorption energy varied from site to site.Accordingly,the adsorption of chloride ions was determined to be local.Berzing [13]measured the adsorption isotherms on corroding Al with Cl 36as a radioactive tracer.The amount of chloride adsorbed w Cl ,was a function of the chloride concentration (Cl)and time according to the equation:
log w Cl 0X 64 log Cl log t À7X 8遥控直升机制作
where w Cl is expressed as g cm À2,(Cl)as mol À1and t (min).Chloride adsorption was primarily localized to the corroding pit sites.The adsorption of chloride ions on the passive Al was found to increase linearly with the potential.Addition of nitrate or sulfate delayed but did not prevent the uptake of chloride.The log of delay time was a function of the log chloride concentration and inhibitor species.It was concluded that a corroding aluminum surface has a variety of adsorption sites with di erent adsorption properties;only a minority of these sites are active for pitting corrosion.According to the authors the results suggest that there is no threshold for the chloride concentration below which pitting will not occur.Similarly,the presence of an inhibitor will delay but not prevent the onset of pitting.Using the XPS method,Augustynski [15]found chloride ions incorporated at the corrosion potential in the ®rst 10±15A of the oxide/solution interface.At more anodic potentials a signi®cant a
mount of chloride is also present inside the oxide ®lm.An increased adsorption of Cl Àwas observed at high temperatures probably as a result of increased porosity of the passive ®lm.
3.Metastable alloys
There are not many metals which dissolve in aluminum above a few atomic percent,those that do dissolve are Mg,Zn and Li.The equilibrium solubility of copper,one of the most important elements in aluminum alloys,is only 5.7at%.When copper is in a solid solution pitting is inhibited [16].None of the existing commercial alloys have very good resistance to localized corrosion.Therefore,there have been several e orts to produce ``stainless''aluminum with a high resistance to localized corrosion in a chloride solution.To obtain this goal,chemically and physically homogeneous supersaturated Al alloys were produced by adding di erent elements exhibiting good resistance to pitting.It is expected that alloying elements which may be incorporated into the passive ®lm will in¯uence not only the chemical properties but the electronic structure of the ®lm as well.These metastable alloys were produced only in the form of thin ®lms (using nonequilibrium methods)which are useless for technical purposes.However,many papers dealing with nonequilibrium Al-alloys provide interesting fundamental information on the e ect of alloying elements on pitting.The pitting potential of aluminum can be substantially increased by the addition of such
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1746
elements as W,Ta,Mo,and Cr [17±22].Fig.1taken from ref.[21]shows anodic polarization of di erent Al-alloys in 0.1M KCl.As can be seen,all of these elements increase the pitting potential in a neutral chloride solution.In the case of Al-9%W the pitting potential is shifted to more than IV in a more positive direction.Surface analyses were made for oxides on Al±Ta,Al±Cr [19],Al±W [21]and Al±Mo [23].The passive ®lm on Al±Mo,Al±Cr,Al±Ta has a considerable amount of oxidized solute (5±10%)but only 0.1±0.2%of W was detected in the oxide on the Al±W alloy.Frankel et al.[24]studied pitting of thin (1000±2000A )sputter deposited Al-binary alloys (mainly Al±Nb,Al±Mo and Al±Cr).Measurements of the pitting and repassivation potential show that stable pits initiate at potentials only about 30mV higher than they repassivate (Fig.2).Hence the contribution of the passive ®lm in inhibiting pitting is low,and pit growth is dependent upon events occurring within pits [24].Other evidence that incorporation of alloying elements into the passive ®lm does not play a fundamental role in pitting is given by Krishnakumar and Smialowska [25].They studied passivation characteristics of a sputtered metastable Al±Cr and Al±Ta alloy by electrochemical impedance spectroscopy.It
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Fig.1.Anodic polarization curves for Al alloys in deaerated 0.1M NaCl solution [21].Z.Szklarska-Smialowska /Corrosion Science 41(1999)1743±17671747
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