Elektrohemijsko taloženje i karakterizacija zaštitnih prevlaka Zn-Mn legura

Abstract

Ispitivan je uticaj vrste i sastava rastvora za taloženje i gustine struje taloženja na svojstva elektrohemijski dobijenih prevlaka Zn-Mn legura na čeliku. Korišćeni su rastvori gde su kao izvor jona metala bili hloridi (hloridni rastvori), sulfati (sulfatni rastvori) i hloridi uz visoku koncentraciju kalijum-pirofosfata kao kompleksirajućeg reagensa (pirofosfatni rastvori). Prevlake su bile taložene gustinama struje u opsegu između 20 i 300 mA cm-2. Uticaj parametara taloženja na hemijski sastav Zn-Mn legura je ispitan atomskom apsorpcionom spektroskopijom i energetskom disperzionom atomskom analizom, fazni sastav legura je određen rendgenskom difrakcijom, dok je morfologija površine prevlaka ispitana optičkom, elektronskom i mikroskopijom atomskih sila. Koroziona postojanost prevlaka Zn-Mn legura je ispitivana u slanoj komori kao i u 0,5 mol dm-3 rastvoru NaCl, pomoću više elektrohemijskih tehnika. Pokazano je kako rastvor za taloženje i gustina struje taloženja imaju veliki uticaj na svojstva i koroziono ponašanje Zn-Mn legura. Sadržaj Mn u prevlaci raste sa porastom gustine struje taloženja, i najveći sadržaj Mn (36 at.%) je postignut taloženjem iz hloridnog rastvora. Rendgenskom difrakcijom je potvrđeno prisustvo tri kristalne faze, i to η, ε- ili β1 faze, zavisno od uslova taloženja legure. Ispitan je uticaj dva specifična dodatka za taloženje iz grupe aromatičnih aldehida, i utvrđeno je da ove supstance inhibiraju izdvajanje vodonika na katodi i rast dendrita u prevlaci, i povećavaju sadržaj Mn u Zn-Mn legurama. Utvrđeno je da za svaki od tri vrste rastvora za taloženje (hloridni, sulfatni i pirofosfatni) postoji optimalna oblast gustina struje taloženja u kojoj se dobijaju koroziono najpostojanije prevlake. To su gustine struje koje daju prevlake sa dovoljnim sadržajem Mn koji omogućava stvaranje kompaktnog pasivnog sloja na korodirajućoj površini, ali istovremeno prevlake taložene tim gustinama struje imaju homogenu morfologiju površine. Optimalni opseg gustina struje je: za hloridni rastvor 20 – 80 mA cm-2, za sulfatni 20 – 40 mA cm-2 i za pirofosfatni 30 – 60 mA cm-2...

The influence of a solution type and composition, as well as deposition current density, on the properties of electrodeposited Zn-Mn alloys on steel was investigated. Three solution types were used, where the source of metal ions were chlorides (chloride solutions), sulphates (sulphate solutions), and chlorides with a high concentration of potassium pyrophosphate, used as complexing agent (pyrophosphate solutions). The deposition current densities were ranging from 20 to 300 mA cm-2. The chemical composition of Zn-Mn alloys was investigated by atomic absorption spectrometry and energy dispersive X-ray analysis, the phase composition was determined by X-ray diffraction, while the coating morphology was investigated by optical, scanning electron, and atomic force microscopy. The corrosion behaviour of the Zn-Mn coatings was investigated in the salt spray chamber and in 0,5 mol dm-3 NaCl solution, by using several electrochemical techniques. The solution type and the deposition current density show high influence on the properties and corrosion behaviour of the Zn-Mn deposits. The Mn content increases with the current density increase, and the highest content of 36 at.% Mn can be achieved by the deposition from chloride solution. Three crystalline phases were identified by X-ray diffraction, namely η, ε or β1 phases, depending on the deposition conditions. The influence of the two brightening agents, which belong to the aromatic aldehydes, was also investigated, and it was shown that they not only inhibit hydrogen evolution reaction and dendrite growth, but also increase the Mn content in the Zn-Mn alloys. For each of the solution types (chloride, sulphate or pyrophosphate), there is an optimal deposition current density range which enables the formation of the Zn-Mn coatings with the highest corrosion stability. In this current density range, the obtained coatings possess enough Mn to enable the formation of passive layer on the corroding surface, and at the same time, the coatings obtained in this current density range are with homogeneous surface morphology. The optimal current density range is 20 – 80 mA cm-2 in chloride, 20 – 40 mA cm-2 in sulphate, and 30 – 60 mA cm-2 in pyrophosphate solution...