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Different Substrates Require Different Treatments To Provide Adherent Coatings

This document discusses different substrate pretreatment techniques needed to provide adherent electrodeposited coatings for various materials. It presents two examples: (1) Pickling uranium in nitric acid and nickel chloride solution increases its surface area and creates sites for mechanical interlocking with electrodeposited coatings. This leads to failure in the coating rather than at the interface. (2) Mechanically roughening tantalum through sandblasting followed by anodic etching improves adhesion of electrodeposited nickel, copper and silver coatings. The roughening creates pits approximately 50-75 micrometers deep, improving adhesion compared to anodic etching alone.

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53 views3 pages

Different Substrates Require Different Treatments To Provide Adherent Coatings

This document discusses different substrate pretreatment techniques needed to provide adherent electrodeposited coatings for various materials. It presents two examples: (1) Pickling uranium in nitric acid and nickel chloride solution increases its surface area and creates sites for mechanical interlocking with electrodeposited coatings. This leads to failure in the coating rather than at the interface. (2) Mechanically roughening tantalum through sandblasting followed by anodic etching improves adhesion of electrodeposited nickel, copper and silver coatings. The roughening creates pits approximately 50-75 micrometers deep, improving adhesion compared to anodic etching alone.

Uploaded by

Jorge Arrieta
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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56 Electrodeposition

Table 3- Different Substrates Require Different Treatments


to Provide Adherent Coatings

steel stainless steels titanium


copper aluminum molybdenum
brass beryllium tungsten
magnesium niobium
plastics tantalum
glass

Table 4- Thickness of Oxide Films

A1203 18 28
Fe203 40 28
NiO 6,lO 28,29
Ta205 16 30,31
300 Series
StainlessSteel 20-1 00 30,31

substrates for coating. They include pickling in concentrated acids,


mechanical roughening, intermediate strike coatings, displacement films,
anodic oxidation, heating after plating, plasma/gas etching and physical
vapor deposition using augmented energy (ion plating). Examples of each
technique, itemized in Table 5 , will be presented in the following sections.

A. Pickling in Concentrated Acids

Uranium is a good example to use to demonstrate how pickling in


concentrated acids can help provide adhesion in some cases. If proper
procedures are used, it is possible to obtain suitable mechanical adhesion
between uranium and electrodeposited coatings. The most, successful
techniques involve chemical pickling of the uranium in concentrated acid
solution containing chloride ions (e.g., 500 g/i nickel chloride plus 340 ml/l
nitric acid), followed by removal of the chloride reaction products in nitric
acid before plating. This treatment does nothing more than provide a much
Adhesion 57

Techniaue ed in t.~&)

Pickling in concentrated acids Etching uranium in nitric acidhickel


chloride solution

Mechanical roughening Tantalum plated with nickel

Intermediate strike coatings Wood's nickel strike; "glue" coatings on


glass

Displacement films Zinc films on aluminum and beryllium

Anodic oxidation Phosphoric acid anodizing of aluminum

Heating after plating Electroless nickel on aluminum; nickel on


Zircaloy-2

Plasmdgas etching Plating on plastics

Physical vapor deposition Coatings on tungsten, molybdenum and


(ion plating) titanium

Miscellaneous Interface tailoring, oxide formation, partial


pressure of gases, reactive ion mixing,
phase-in deposition

increased surface area with many sites for mechanical interlocking or


"interfingering" of the deposit. However, extremely good adherence can be
obtained. Figure 7 shows the roughening and tunneling sites in etched
uranium that provide the mechanical interlocking. Ring shear tests on parts
receiving this type of treatment show failure in the coating rather than at the
interface between the substrate and coating
(32).
58 Electrodeposition

Figure 7: "Interfingering" developed in uranium as a result of etching in


nickel chloride/nitric acid solution prior to nickel plating. Magnification is
300 x.

B. Mechanical Roughening

Tantalum is one of the most difficult metals to coat with an


adherent electrodeposit. Results that have been reported previously are
qualitative in nature, probably because quantitative data simply couldn't be
obtained. Recent data show that by using mechanical roughening followed
by anodic etching, reasonably good adhesion can be obtained. Adherent
deposits of nickel, copper, and silver were obtained on tantalum when the
tantalum was sandblasted and then anodically etched for 20 minutes at 200
A/m2 in a methanol solution containing 2.5 v/o HCl and 2.5 v/o HF
operated at 45OC. Depth of pitting as a result of the sandblasting/etching
process was approximately 50-75 pm (2-3 mils). Peel strength data in Table
6 clearly show the importance of the mechanical roughening (sandblasting)
part of the process. Without the mechanical roughening step, the subse-
quent anodic etch was extremely non-uniform and adhesion was consider-
ably reduced.

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