The influence of electrochemically deposited zinc alloys on steel on the protective properties of cathaporetic epoxy coatings

Abstract

Prior to cathaporetic epoxy coating deposition steel surface was modified by chemical deposition of hot dip galvanized steel and by electrochemical deposition of zinc alloys with iron group of elements (Ni, Co and Fe). The influence of the deposition solution and different deposition parameters on the chemical content, phase structure and corrosion properties of zinc alloys was examined. It was shown that the composition of the plating bath its temperature and the deposition current density have a big influence on the corrosion behavior of zinc alloys, as well as on the corrosion stability of the protective system zinc alloy / epoxy coating. Among all investigated zinc alloys Zn-Ni alloy deposited from chloride bath at 2 A dm2 exhibited the best corrosion properties. The corrosion stability of the protective system zinc alloy / epoxy coating was investigated by determining the values of pore resistance and charge-transfer resistance (obtained from impedance measurements (EIS)). Sorption characteristics were determined by gravimetric measurements and thermal stability by thermo gravimetric analysis. The kinetics of hydrogen evolution on different substrates (the first step of epoxy coating electrode position) was determined, as well as the wet ability of different substrates by polymer solution. The influence of these two factors on the porosity and thermal stability of epoxy coatings was shown. It was shown that epoxy coating on Zn-Ni alloy obtained from chloride bath has the highest corrosion stability among all investigated protective systems (the highest values of pore resistance and charge transfer resistance), while epoxy coating on Zn-Fe alloy has the lowest corrosion stability. It was also shown that corrosion processes on the steel, under the epoxy coating, begin later on Zn-Ni.

Površina čelika je modifikovana elektrohemijskim taloženjem legura cinka sa elementima grupe gvožđa (Ni, Сo i Fe) i prevlakom cinka, pre nanošenja kataforetske epoksidne prevlake. Ispitivanje uticaj sastava rastvora za taloženje i raznih parametara taloženja na hemijski i fazni sastav legura cinka, kao i na gustine struje korozije legura. Pokazano je da sastav rastvora za taloženje, njegova temperatura i gustina struje taloženja imaju veliki uticaj na koroziono ponašanje legura cinka, kao i na korozionu stabilnost zaštitnih sistema legura cinka / epoksidna prevlaka na čeliku. Od svih ispitivanih legura Zn-Ni legura dobijena taloženjem iz hloridnog rastvora sa 2 A dm2 ima najmanju gustinu struje korozije. Zaštitna svojstva sistema legura cinka / epoksidna prevlaka na čeliku su određivana merenjem vrednosti otpornosti elektrolita u рorama prevlake i otpornosti prenosu naelektrisanja kroz graničnu površinu metal-elektroli (metoda spektroskopije elektrohemijske impedancije (SEI)). Sorpcione karakteristike su određivane gravimetrijskim merenjima, a termička stabilnost termogravimetrijskom analizom. Ispitivane su brzine izdvajanja vodonika, kao prvog stupnja u kataforetskom taloženju epoksidne prevlake, na različitim supstratima, kao i kvašljivost supstrata rastvorom polimera. Pokazano je kako ova dva faktora zajedno utiču na poroznost i termičku stabilnost epoksidnih prevlaka. S obzirom na najveće vrednosti otpornosti elektrolita u рorama prevlake i otpornosti prenosa naelektrisanja kroz graničnu površinu metal-elektrolit od svih ispitivanih legura najbolja svojstva pokazuje legura Zn-Ni a najlošija Zn-Fe. Pokazano je, takođe, da se početak korozionih procesa na čeliku, ispod prevlaka legura, najkasnije javlja u slučaju Zn-Ni legure.