Characterization and corrosion properties of electrodeposited Ni-W alloys

Abstract

Ni-W alloys electrodeposited from citrate solution consist of three different phases, which are all present in high W-content alloys: a solid solution of W in a Ni matrix; an intermetallic compound Ni4W, as well as another solid solution of W in Ni with a W content higher than 20 mol. %. XPS analysis revealed that the alloys were covered with a surface layer of complex structure containing the pure metals Ni and W, Ni(OH)2 andWO3, on the very surface as well as some partially reduced oxideWO2.72 (most probably a solid solution ofWO2.72 in Ni) and tungsten carbide in the layer underneath. It is highly likely that some of the oxide species in the layer act as intermediates in the cathodic deposition process. Identifying these species should be the clue to a more detailed understanding of the mechanism of induced deposition of W than has been achieved so far. Corrosion of Ni-W alloys in sulfuric acid solution at OCP, occurs by the preferential dissolution of nickel from the surface layer. The longer the corrosion process lasts, the more the surface behaves like pure W. The lowest initial corrosion rates were recorded with alloys rich in W, but after aging at OCP the lowest corrosion c.d. was found for the Ni-W alloy with the most homogenous phase structure, consisting of the solid solution only.

Elektrohemijski taložene legure Ni-W iz citratnog rastvora sadrže tri različite faze, koje su prisutne pri visokom sadržaju W u leguri: čvrst rastvor W u Ni; intermetalno jedinjenje Ni4Wi čvrst rastvor Wu Ni sa sadržajem W većim od 20 mol %. XPS analizom je pokazano da su legure pokrivene slojem složene strukture. Ovaj sloj sadrži čiste metale Ni i W, Ni(OH)2 i WO3, na samoj površini. U sloju ispod same površine nađeni su i delimično redukovan oksid WO2,72 (najverovatnije čvrst rastvor WO2,72 u Ni) i volfram-karbid. Vrlo je verovatno da neka od oksidnih vrsta u sloju učestvuje kao intermedijer u procesu katodnog taloženja. Identifikacija te vrste mogla bi doprineti boljem razumevanju mehanizma indukovanog taloženja volframa. Na potencijalu otvorenog kola u sumpornoj kiselini korozioni proces se odigrava tako što se preferencijalno rastvara Ni iz površinskog sloja legure. Ukoliko korozioni proces traje duže utoliko korozione karakteristike postaju sve sličnije onima za čist W. Niže početne brzine korozije dobijene su za legure sa većim sadržajem W. Međutim, nakon stajanja na potencijalu otvorenog kola niže korozione struje su dobijene za Ni-W leguru homogene fazne strukture koja sadrži samo čvrst rastvor.