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|Title: ||Molybdate-based passivation treatments for tin, tinplate and zinc|
|Authors: ||Wilcox, G.D.|
|Issue Date: ||1986|
|Publisher: ||© G.D. Wilcox|
|Abstract: ||The level of toxicity of chromate conversion coating solutions has led
to a search for alternatives. Molybdate has been considered, being a
relatively non-toxic analogue from group (VIA) of the periodic table.
The ability of molybdate to perform as an inhibitor in aerated
conditions is well-reported, although its application in conversion
coatings is less widely documented.
The experimental work reported in this thesis was to a large extent
based on potentiodynamic polarisation which is considered a powerful
tool for evaluating potential coating solutions. The cathodic
polarisation characteristics of zinc in group (VIA) oxy-anion
solutions has been studied quantitatively. The effect of temperature,
pH and aeration has been examined. The film-forming ability of
molybdate-based solutions such as molybdate-orthophosphoric acid (MP),
molybdate-orthophosphoric acid-nitrate and molybdate-orthophosphate
has been examined with respect to a tin, tinplate or zinc substrate.
Films were formed electrochemically (anodically and cathodically) or
by a simple immersion technique. The performance of such coatings has
been studied using classical corrosion tests such as sulphide stain
and salt spray.
The cathodic reduction characteristics of molybdate solutions have
also been examined in detail with respect to possible lower valency
films being produced (possibly as a result of molybdate's complex
electrochemistry) on tinplate, zinc and platinum metal surfaces.
Experimental results have indicated that simple, relatively dilute,
molybdate solutions at pH 3 are capable of imparting good staining
resistance to tinplate in sulphide stain tests. However, other mixed
systems have proved less successful, only molybdate-orthophosphoric
acid providing acceptable protection on tin foil. Simple immersion
techniques in dilute, pH 5 molybdate solutions have provided moderate
protection on zinc in 24-hour salt spray tests.
Inflections exhibited on cathodic polarisation curves of tinplate in
simple molybdate solutions have been attributed to a physical
degradation of the coating as opposed to a reduction step.|
|Description: ||A Doctoral Thesis. Submitted in partial fulfillment of the requirements for the award of Doctor of Philosophy of Loughborough University.|
|Appears in Collections:||PhD Theses (Materials)|
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