Electrochemical methods for corrosion testing of Ce-based coatings prepared on AA6060 alloy by the dip immersion method

Abstract

Dip-immersion is a simple and cost-effective method for the preparation of Ce-based conversion coatings (CeCCs), a promising alternative to toxic chromate coatings, on metal substrates. In this work, CeCCs were prepared on Al-alloy AA6060 from an aqueous solution of cerium chloride at room temperature. The effect of immersion time and post-treatment in phosphate solution on the microstructure and corrosion properties of the coatings was studied. The longer the immersion time was, the thicker but more nonhomogeneous and cracked were the CeCCs. The post-treatment contributed to a sealing of the cracks, as proven by an increase in the corrosion resistance compared with the as-deposited coatings. The CeCCs prepared at longer deposition times and post-treated showed much better corrosion protection than those prepared at shorter deposition times. A detailed electrochemical impedance spectroscopy (EIS) study was undertaken to follow the evolution of the corrosion behaviour of the CeCCs with time of exposure to an aggressive chloride environment (3.5 % NaCl). For the sake of comparison, the EIS properties of bare AA6060 were also investigated. Linear voltammetry was performed to complete the study. The results confirmed the formation of protective CeCCs on the surface of AA6060. However, even CeCCs prepared at longer deposition times and post-treated provided short term protection in the aggressive environment, due to the small thickness of the coating.

Nanošenje prevlaka potapanjem u rastvor je jednostavna i ekonomski prihvatljiva metoda za pripremu konverzionih prevlaka na bazi cerijuma na metalnim supstratima, koje su dobra zamena za toksične hromatne prevlake. U ovom radu prevlake na bazi cerijuma su nanesene na aluminijumsku leguru AA6060 iz vodenog rastvora cerijum-hlorida na sobnoj temperaturi. Ispitivan je uticaj vremena potapanja i uticaj naknadnog tretmana u fosfatnom rastvoru na mikrostrukturu i zaštitna svojstva dobijenih prevlaka. Dužim potapanjem su dobijene deblje ali nehomogene prevlake. Naknadni tretman je povoljno uticao na zatvaranje prskotina, što je u saglasnosti sa izmerenom većom korozionom otpornošću u poređenju sa prevlakama bez tretmana. Prevlake dobijene dužim nanošenjem su pokazale bolju korozionu zaštitu od prevlaka dobijenih kraćim nanošenjem. Koroziona stabilnost prevlaka je ispitivana u agresivnoj hloridnoj sredini (3,5 % NaCl) metodama spektroskopije elektrohemijske impedancije (SEI) i linearne voltametrije. Detaljno je praćeno koroziono ponašanje prevlaka sa vremenom delovanja korozionog agensa. Poređenja radi, SEI analiza je sprovedena i za leguru AA6060 bez prevlake. Rezultati istraživanja potvrđuju da dobijene prevlake na bazi cerijuma predstavljaju dobru korozionu zaštitu leguri AA6060. Međutim, čak i kvalitetnija prevlaka (20 min potapanja i naknadno tretirana u fosfatnom rastvoru) obezbeđuje leguri kratkoročnu zaštitu u agresivnoj sredini usled svoje male debljine.