Making Ti TougherAdvAnces in selecTive plATing on TiTAniuM AerospAce Alloys

By Darrin Radatz, Ani Zhecheva and Sid ClouserSIFCO Applied Surface Concepts

Reprinted From: Products Finishing Magazine

igh strength and low weight, coupled with the ability to readily form a tenacious

surface oxide film, make titanium and its alloys useful in many appli-cations in the aerospace, industrial and medical fields. One of the most useful titanium alloys contains 6% aluminum and 4% vanadium (Ti-6Al-4V).

A limitation of titanium alloys is relatively poor resistance to adhe-sive wear, which results in galling and cold welding, poor fretting behavior and a high coefficient of friction. Tribological performance can be improved by applying sur-face treatments and coatings while retaining the positive attributes of the underlying substrate. Coatings are also applied on titanium for heat reflection, emissivity, corro-sion resistance in hot acidic envi-ronments, conductivity, lubricity, brazing and resizing.

One way to apply coatings to titanium alloys for aerospace and

H

Brush plating is commonly used to repair and recondition aerospace components.

other applications is selective brush plating, a metallization technique that uses a small volume of solution contained in a fabric matrix between the anode and cathode in a plating cell. The small volume of plating solution, coupled with the proximity of the anode and cathode, allows use of brush plating in applications that would be impractical or impossible with other electroplating or coat-ing methods.

surfAce prepArATionTitanium is very reactive and

rapidly forms an oxide film when-ever the metal surface is exposed to air or any environment contain-ing available oxygen. This oxide layer should be removed before electroplating or other surface treatment, but its tenacity makes removal problematic.

A common method to remove oxide from titanium is exposure to

an acid fluoride-containing electro-lyte. The toxicity of fluoride, and the rapidity with which the oxide re-forms after removal, are issues with this method.

Surface roughening can improve coating adhesion, and can be accomplished by abrasion, grit blasting and etching. Surface preparation is key to achieving robust adhesion of any coating to titanium.

BrusH plATing reseArcH SIFCO Applied Surface Concepts

has performed multiple experi-ments on titanium surface prepa-ration and selective plating. We obtained titanium sheets 1.1 mm thick and tubes 0.83 mm thick in three substrate materials: Ti-6Al-4V, Ti-6Al-6V-2Sn and commercially pure Grade 2 titanium. We used the sheets and tubes in the as-received condition and mechanically fin-ished the surfaces using several

techniques. These included dry or wet abrasion with red 3M Scotch-Brite, wire brushing and abrasive blasting with aluminum oxide and silicon carbide.

Chemicals used to formulate pretreatment solutions and plat-ing baths were reagent grade without further purification. We used deionized water for solution makeup and rinsing, and first applied a nickel strike layer using an acid nickel sulfate plating solu-tion and a rectifier capable of being switched rapidly between cathodic and anodic modes.

The anode was a noble metal-coated titanium mesh supported by a polypropylene block. Holes drilled through the block allowed solution to flow through the anode structure onto the workpiece. The anode-to-cathode gap, which was determined by the thickness of the fabric matrix between the anode and cathode, was approximately

scanning electron micrographs of Ti-6Al-4v surface after each process step. Abrasion, anodic etching and cathodic activation result in a dense nickel strike layer with a slightly nodular morphology.

As Received Abraded Etched

Activated Nickel Plated

5 mm (0.2 in.). Fabrics evaluated included red, brown and grey 3M Scotch-Brite, cotton, polyethylene terephthalate and gauze.

Adhesion of the plated metal to the titanium substrate was tested according to AMS 2451A and ASTM B 571. Adhesion was evalu-ated using the chisel-knife, tape, quench and bend tests. Titanium sheets were bent 180° around a diameter equal to the thickness of the sheet in the bend test. Parts were quenched from 250°C (480°F) into room-temperature water, and the tape was pulled with a quick motion at a 180° angle to the surface. A sharp cold chisel was used to penetrate the coating. A good result in each of these tests shows no separation of plating from the basis metal.

The tensile adhesion of a 50-µm thick nickel coating on Ti-6Al-4V was measured following ASTM C 633 and using adhesive EC2086,

then examining the interface between titanium and plating with an inverted metallurgical micro-scope. We used a stylus profilometer to measure roughness.

One objective of the work was to develop a fluoride-free electrolyte for surface preparation of titanium. Accordingly, we evaluated sulfuric, nitric, sulfuric/nitric mixtures, sulfamic, ammonium bifluoride and phosphoric acid at various concentrations. Visible oxide films formed during anodic treatment in some of the acid solutions which could not be cathodically reduced. Presence of the film coincided with poor deposit adhesion.

iniTiAl resulTsResults showed that the titanium

alloy, acid type and concentration and pretreatment conditions all impacted adhesion of an electrode-posited coating on titanium and alloy substrates.

Our experience selectively plat-ing a nickel deposit on titanium Grade 2 is representative of the issues involved with obtaining adherent coatings on titanium. The Grade 2 titanium surface was blasted with 180-grit silicon carbide and activated before plating. The test coupon survived tape and bend tests, however, separation between the nickel deposit and the substrate observed in cross-section, considered to involve the titanium oxide film, makes adhesion sus-pect. Nickel brush plated over the oxide film had poor adhesion in localized areas.

We then tried mechanical meth-ods to improve adhesion by increas-ing the substrate surface area and exposing a fresh, clean titanium surface. Mechanical working of the surface by abrasion with grinding media, wire brushes or by blasting with silicon carbide or wet or dry alumina increased surface area, and deposit adhesion improved versus an unworked surface. Adhesion was still not high enough to routinely survive the 180° bend test.

Acid eTcHingTitanium undergoes active dis-

solution in strong acids, and we used hydrochloric and sulfuric acids to increase substrate surface area and remove the oxide film. The substrate surface was matte after the acid etches. Hydrogen evolution during active dissolu-tion in the acids indicated that the titanium surface was free of oxide. Low deposit adhesion was observed on the chemically-etched titanium surfaces.

We then undertook to identify an electrochemical treatment method with the capability to increase the substrate surface area in a con-trolled manner and provide an oxide-free surface that enabled good deposit adhesion. The result-ing electrochemical treatment includes both an electrolyte and

cross section through a nickel strike/brush copper/brush nickel coating on Ti-6Al-4v shows no delamination between layers. This type of coating is often used to repair surface defects.

Nickel Strike/Plate

Ti 6-4 Base

Acid Nickel

Acid Copper

35 µm

350 µm

60 µm

Table 2 average tensile adhesion of nickel deposits on Ti-6al-4V

Surface Type of acid Nickel Solution adhesion, psi

Machined Sulfate 7,400

Blasted Sulfate 6,500

Blasted Sulfamate 6,600

EC2086 epoxy 13,000

an anodic/cathodic etch/activate methodology to promote micro-etching of the titanium surface to increase surface area and reduce the surface oxide.

Treatments anodically polarized the titanium surface to microetch/roughen it. This was immediately followed by a cathodic treatment in the same solution to reduce oxide, then introducing a nickel strike plating solution. Treatments were carried out under potential control. We identified mixtures of sodium chloride, nickel salts, and hydrochloric and citric acids that met the objectives of an etch/acti-vate electrolyte. The nickel strike and buildup were plated using the

acid nickel sulfate brush plating solution. Nickel plated readily into the microroughness features.

Adhesion was checked by tape and bend tests, and the nickel deposit did not separate from the Ti-6Al-4V substrate. Thin panels of Ti-6Al-4V and Ti-6Al-6V-2Sn broke in one 180° bend without separa-tion or peeling of the deposit. Grade 2 titanium can be bent a full 180° without breaking. The bend test was more rigorous in revealing low adhesion than the tape test.

Several factors contribute to the excellent adhesion: mechanical interlocking, increased surface area and lack of an oxide film. These three attributes were gen-erated during the brush plating process. Brush plating is particu-larly suited for generating these attributes because of the small volume of electrolyte, close contact between the anode and the cath-ode, and the rapidity with which

electrolytes can be switched from activation to strike plating.

The plating procedure used to make this deposit is given in Table 1. Important considerations for this process are to keep the titanium under potential control at all times, keep the plated area 100% covered by the wrapped anode, use rapid

Table 1 brush plating procedure for nickel coatings on Ti-6al-4V

Operation Material Conditions

Abrade Scotch-Brite Wet with Etch/Activate Solution

Etch Etch/Activate 14 V anodic, 10 sec

Activate Solution 4–8 V cathodic, 1 min

Strike Plate Acid Nickel 8–18 V cathodic, 0.078 A∙hr/cm2

Future research work should continue to develop principles for good adhesion of plated deposits to titanium alloys

switching from anodic to cathodic, allow no rinsing between steps and do not reuse the solution.

Tests produced further under-standing of both the chemistry and mechanical processing needed to activate the titanium surface. Scale on the rolled panels had to be removed mechanically, and abrasion of panels was a require-ment for good adhesion in the absence of blasting. Grit blasting, wire wheel abrasion, grinding and abrasive pad rubbing were effective in preparing the surface for the

etch/activate process. A machined surface requires little in the way of mechanical work before etch/activate. Deep scratches should be avoided during abrasion—they can develop pits in the anodic etch.

Rinsing is not included between the process steps in Table 1 because, when the titanium surface is

removed from the electrolyte, the oxide film readily forms in either air or water. This interferes with adhesion between the deposit and the substrate. The solutions we used in this work are compatible and a water rinse between the process steps is not required. A final rinse removes electrolyte components after plating is complete.

Scanning electron microscope images of the titanium surface after each process step clearly show roll-ing lines in the as-received panels. Abrasion replaces the rolling lines

with a new mechanical finish, and the anodic etch further increases surface area. At high magnifica-tion, micron- and sub-micron-size etch features are observed in the surface. These represent good anchoring points for subsequent deposits, and the subsequent nickel strike layer is dense with a slightly nodular morphology.

surfAce rougHnessWe measured roughness changes

produced by each surface treat-ment using a mechanical profi-lometer. Although the panels had a rolling direction, average rough-ness was similar in both directions. Scotch-Brite abrasion decreased roughness slightly, but roughness was increased by anodic etching. Microroughness induced by anodic etching is a key component in attaining good deposit adhesion to the Ti-6Al-4V panel The anodic etch removes 0.0089 g/cm2 (0.0008 in.) of titanium.

The activation step designed to reduce the oxide formed during anodic etching reduces the surface roughness. The average roughness (Ra) in both directions after activa-tion was somewhat lower than Ra after etching. We determined that roughness of the plated layer was more a function of the nickel brush plating parameters.

We also used a scanning electron microscope to examine the inter-face between the deposit and tita-nium alloy substrate. Microrough-ness and the lack of a bulk oxide film on the titanium was evident. There was no sign of separation between the two metals.

oTHer evAluATionsTensile adhesion of brush-plated

nickel to Ti-6Al-4V was measured following ASTM C 633. Average values measured on multiple stubs are reported in Table 2. The surface of the titanium alloy stubs was pretreated by machining or

SiC grit-blasting, then abrading, etching and activating using the process in Table 1. A 50-µm thick nickel deposit was plated from two acid electrolytes. The failure mode in all specimens was adhesive, at the nickel coating - titanium interface. Of note, the grit-blasted surface did not increase the tensile adhesion versus the machined surface. The bond strength did not depend on the type of acid nickel plating solution used for the strike plate and buildup.

Hydrogen embrittlement was tested according to General Motors Engineering Standard GM3661P, in which a sustained test load in bending of 85% of the yield is applied to the part for 24 hr. Ti-6-Al-4V coupons predrilled with a 0.25-in. diameter hole for a stress riser were plated on one side with 15 µm (0.0006 in.) of nickel using the process in Table 1. Six coupons were plated; three were tested as-plated and three were heat treated at 190°C (375°F) for 24 hr prior to test.

All samples were satisfactory for hydrogen embrittlement to specifi-cation GM3661P—that is, no failure or cracking was observed on the any of the coupons. The effect of hydrogen embrittlement was thus considered negligible.

This technology also performed well with Ti-6Al-6V-2Sn alloy, and deposit adhesion was satisfactory. However, the procedure does not provide a deposit with adequate adhesion on Grade 2 titanium. Deposits on Grade 2 generally passed tape but failed bend tests. A variety of processes and chem-istries did not microroughen and activate Grade 2 titanium.

The difference appears during anodic etching. The alloys micror-oughen, while a thin, visible oxide film forms on Grade 2 and persists through cathodic activation. Com-mercially pure titanium appears to passivate more quickly than

the alloys, and this compromises adhesion. It is speculated the alloying constituents are more prone to dissolution during the anodic etch than titanium, lead-ing to the microrough surface morphology.

oTHer WorkWe also developed a procedure

for electroplating a duplex coating on Ti-6Al-4V. The substrate was prepared by the process in Table 1, applying a 35-µm thick (0.0014 in.) nickel layer. The nickel-plated Ti-6Al-4V was brush plated using an acid copper electrolyte to a thickness of 350 µm (0.014 in.). This structure was then machined to reduce the copper roughness. No delamination of copper from nickel or nickel from Ti-6Al-4V was observed because of machining stresses. The machined copper was then brush plated with a 60-µm thick (0.0024-in.) nickel cap. This duplex coating structure is often used to repair surface defects.

Future research work should con-tinue to develop principles for good adhesion of plated deposits to tita-nium alloys, and identify a process to deposit coatings with improved adhesion on Grade 2 titanium. Deposition of other materials with better wear resistance than titanium will also be investigated.n

For information from SIFCO Applied Surface Concepts, phone 216-524-0099 or go to www.sifcoasc.com.

avoid Nickel Plating lossesTo learn how to minimize loss of nickel metal in your plating operation, go to pfon-line.com/articles/090503.html.

LearnMORE

Reprinted from PRODUCTS FINISHING Magazine

December 2009 and Copyright © 2009 by Gardner Publications, Inc., 6915 Valley Ave., Cincinnati,

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