* E. Slavcheva Central Laboratory of Electrochemical Power Sources ± Bulgarian Academy of Sciences, Akad. G.Bonchev Str., Bl.10, 1113 Sofia (Bulgaria) G. Schmitt Laboratory for Corrosion Protection, University of Applied Sciences, Iserlohn, Frauenstuhlweg 31, D-58644 Iserlohn (Germany) Screening of new corrosion inhibitors via electrochemical noise analysis Auswahluntersuchungen von neuen Korrosionsinhibitoren mit Hilfe der elektro- chemischen Rauschanalyse E. Slavcheva* and G. Schmitt Dedicated to Prof. Dr. Gu Ènter Schmitt on the occasion of his 60th birthday The inhibition efficiency of a series of organic compounds againstcorrosionofmagnesiumalloyAZ91ina50wt.-%aqueous solutions of ethylene glycol has been studied using polarisation curves and electrochemical noise (ECN) measurements. The com- parative analysis of the results obtained has demonstrated unam- biguouslythatthemethodofECNisaneffectivetoolforscreening ofnewcorrosioninhibitorswhichofferssomeadvantagescompar- ingtothecommonelectrochemicaltechniques.Theveryfastmeas- uringrateenablestoinstantaneouslyregisterandmonitortherateof thecorrosionprocessandchangesinitsmechanisminpresenceof the organic additives. The measurements are performed at freely corroding ªidenticalº electrodes without perturbation from the ex- ternal signals, which could lead (as in the polarisation measure- ments), to changes in the surface structure, the adsorption of the inhibitor etc. The statistical analysis of the noise data (in terms ofstandarddeviations)inthetimedomainshowsthatthenoisein- tensityofthesystemcorrelateswellwiththeintensityofthecorro- sionprocess.Thetotalamountofthechargesexchangedduringthe tests is a quantitative measure for the inhibiting effect of the sub- stances under study, while the spectral analysis of the noise in the frequencydomaincanbeusedtocharacterisethetypeofthecorro- sion attack. DieWirksamkeiteinerReiheorganischerVerbindungenalsKor- rosionsinhibitoren fu Èr die Magnesiumlegierung AZ91 in einer 50 Mass.-% wa Èssrigen Ethylenglykol-Lo ÈsungwurdedurchPolarisati- onsmessungen und elektrochemische Rauschanalyse (ECRA) stu- diert. Ein Vergleich der jeweils erhaltenen Resultate beweist ein- deutig, daû die ECRA eine sehr effektive Methode darstellt, Aus- wahluntersuchungen fu Èr neue Korrosionsinhibitoren durchzufu Èh- ren, und einige Vorteile gegenu Èber herko Èmmlichen elektrochemi- schenMethodenaufweist.EinsehrschnelleMeûdatenaufnahmeer- laubt die unmittelbare Registrierung und U È berwachung der Ge- schwindigkeit des Korrosionsprozesses und von A È nderungen des Korrosionsmechanismus in Gegenwart organischer Additive. Die Messungen werden an frei korrodierenden, ,identischen` Elektro- den ohne Sto Èrung durch externe Signale durchgefu Èhrt, welche ± wiez.B.beiPolarisationsmessungen±zuVera ÈnderungenderOber- fla Èchenstruktur,derAdsorption,etc.fu Èhrenko Ènnen.Diezeitaufge- lo Èste statistische Auswertung der Rauschdaten in Form von Stan- dardabweichungenzeigt,daûdieRauschintensita ÈtderSystemesehr gutmitderenKorrosionsintensita Ètkorreliert.DieGesamtheitderin einer Rauschmeûzeit zwischen den Elektroden ausgetauschten La- dungen ist ein quantitatives Maû fu Èr den Inhibitionseffekt unter- suchter Substanzen. Die spektrale Analyse der Rauschdaten in derFrequenzdomainekannzurCharakterisierungderArtdesKor- rosionsangriffs verwendet werden. 1 Introduction Mgalloysareattractivemetallicmaterials,whichcombine very low density with high strength and stiffness. The proper alloying of pure Mg results in high strength/weight ratios of the alloys and due to the low gravity of Mg (1.74) its alloys weightabout25%lessthantheirAlcounterpartsatequalme- chanicalproperties.AnotherfeaturesforwhichMgalloysare noted are their ease of machining, good casting abilities and highdampingcapacity.RecentlyMgalloysareconsideredas a promising alternative to the Al alloys, which still are dom- inatingconstructionmaterialinautomobileandaerospacein- dustry.ThecorrosionresistanceofMgalloysdependsconsid- erably on film formation and therefore, it varies essentially with the medium to which the alloy is exposed. In general, Mg alloys are resistant in ordinary industrial atmospheres, while aqueous solutions may cause serious corrosion attack, whichdependsnotonlyonthesolutebutalsoonthevolume, movement and temperature of the liquid. Many organic li- quids like aliphatic and aromatic hydrocarbons and their de- rivativesarequiteinerttoMganditsalloyswhilesubstances, which contain reactive polar groups, are aggressive towards them [1]. Magnesium dissolution in aqueous environment generally proceeds by electrochemical reaction with water producing Materials and Corrosion 53, 647±655 (2002) Screening of new corrosion inhibitors 647 Ó 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0947-5117/02/0909-0647$17.50.50/0