Reprinted From: Pr o d u c t s Fi n i s h i n g Magazine Making Ti Tougher Advances in selective plating on titanium aerospace alloys

By Darrin Radatz, Ani Zhecheva and Sid Clouser SIFCO Applied Surface Concepts igh strength and low weight, coupled with the ability to H 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

Brush plating is commonly used to repair and recondition aerospace components. As Received Abraded Etched

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. Activated Nickel Plated

other applications is selective an acid fluoride-containing electro- techniques. These included dry or brush plating, a metallization lyte. The toxicity of fluoride, and wet abrasion with red 3M Scotch- technique that uses a small volume the rapidity with which the oxide Brite, wire brushing and abrasive of solution contained in a fabric re-forms after removal, are issues blasting with aluminum oxide and matrix between the anode and with this method. silicon carbide. cathode in a plating cell. The Surface roughening can improve Chemicals used to formulate small volume of plating solution, coating adhesion, and can be pretreatment solutions and plat- coupled with the proximity of the accomplished by abrasion, grit ing baths were reagent grade anode and cathode, allows use of blasting and etching. Surface without further purification. We brush plating in applications that preparation is key to achieving used deionized water for solution would be impractical or impossible robust adhesion of any coating to makeup and rinsing, and first with other electroplating or coat- titanium. applied a nickel strike layer using ing methods. an acid nickel sulfate plating solu- Brush Plating Research tion and a rectifier capable of being Surface Preparation SIFCO Applied Surface Concepts switched rapidly between cathodic Titanium is very reactive and has performed multiple experi- and anodic modes. rapidly forms an oxide film when- ments on titanium surface prepa- The anode was a noble metal- ever the metal surface is exposed ration and selective plating. We coated titanium mesh supported to air or any environment contain- obtained titanium sheets 1.1 mm by a polypropylene block. Holes ing available oxygen. This oxide thick and tubes 0.83 mm thick in drilled through the block allowed layer should be removed before three substrate materials: Ti-6Al-4V, solution to flow through the anode electroplating or other surface Ti-6Al-6V-2Sn and commercially structure onto the workpiece. The treatment, but its tenacity makes pure Grade 2 titanium. We used the anode-to-cathode gap, which was removal problematic. sheets and tubes in the as-received determined by the thickness of the A common method to remove condition and mechanically fin- fabric matrix between the anode oxide from titanium is exposure to ished the surfaces using several and cathode, was approximately Our experience selectively plat- ing a nickel deposit on titanium 60 µm Grade 2 is representative of the issues involved with obtaining adherent coatings on titanium. Acid Nickel The Grade 2 titanium surface was blasted with 180-grit silicon carbide Acid Copper and activated before plating. The test coupon survived tape and 350 µm 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 35 µm oxide film had poor adhesion in localized areas. We then tried mechanical meth- Nickel Strike/Plate ods to improve adhesion by increas- ing the substrate surface area and Ti 6-4 Base exposing a fresh, clean titanium surface. Mechanical working of the Cross section through a nickel strike/brush copper/brush nickel coating on surface by abrasion with grinding Ti-6Al-4V shows no delamination between layers. This type of coating is often media, wire brushes or by blasting used to repair surface defects. with silicon carbide or wet or dry alumina increased surface area, and deposit adhesion improved versus an unworked surface. Adhesion was 5 mm (0.2 in.). Fabrics evaluated then examining the interface still not high enough to routinely included red, brown and grey 3M between titanium and plating with survive the 180° bend test. Scotch-Brite, cotton, polyethylene an inverted metallurgical micro- terephthalate and gauze. scope. We used a stylus profilometer Acid Etching Adhesion of the plated metal to to measure roughness. Titanium undergoes active dis- the titanium substrate was tested One objective of the work was to solution in strong acids, and we according to AMS 2451A and develop a fluoride-free electrolyte used hydrochloric and sulfuric ASTM B 571. Adhesion was evalu- for surface preparation of titanium. acids to increase substrate surface ated using the chisel-knife, tape, Accordingly, we evaluated sulfuric, area and remove the oxide film. quench and bend tests. Titanium nitric, sulfuric/nitric mixtures, The substrate surface was matte sheets were bent 180° around a sulfamic, ammonium bifluoride after the acid etches. Hydrogen diameter equal to the thickness and phosphoric acid at various evolution during active dissolu- of the sheet in the bend test. concentrations. Visible oxide films tion in the acids indicated that Parts were quenched from 250°C formed during anodic treatment in the titanium surface was free of (480°F) into room-temperature some of the acid solutions which oxide. Low deposit adhesion was water, and the tape was pulled with could not be cathodically reduced. observed on the chemically-etched a quick motion at a 180° angle to Presence of the film coincided with titanium surfaces. the surface. A sharp cold chisel was poor deposit adhesion. We then undertook to identify an used to penetrate the coating. A electrochemical treatment method good result in each of these tests Initial Results with the capability to increase the shows no separation of plating from Results showed that the titanium substrate surface area in a con- the basis metal. alloy, acid type and concentration trolled manner and provide an The tensile adhesion of a 50-µm and pretreatment conditions all oxide-free surface that enabled thick nickel coating on Ti-6Al-4V impacted adhesion of an electrode- good deposit adhesion. The result- was measured following ASTM C posited coating on titanium and ing electrochemical treatment 633 and using adhesive EC2086, alloy substrates. includes both an electrolyte and an anodic/cathodic etch/activate Table 1 Brush plating procedure for nickel coatings on Ti-6Al-4V methodology to promote micro- etching of the titanium surface to Operation Material Conditions increase surface area and reduce Abrade Scotch-Brite Wet with Etch/Activate Solution the surface oxide. Treatments anodically polarized Etch Etch/Activate 14 V anodic, 10 sec the titanium surface to microetch/ Activate Solution 4–8 V cathodic, 1 min roughen it. This was immediately followed by a cathodic treatment Strike Plate Acid Nickel 8–18 V cathodic, 0.078 A∙hr/cm2 in the same solution to reduce oxide, then introducing a nickel electrolytes can be switched from etch/activate process. A machined strike plating solution. Treatments activation to strike plating. surface requires little in the way were carried out under potential The plating procedure used to of mechanical work before etch/ control. We identified mixtures of make this deposit is given in Table activate. Deep scratches should be sodium chloride, nickel salts, and 1. Important considerations for this avoided during abrasion—they can hydrochloric and citric acids that process are to keep the titanium develop pits in the anodic etch. met the objectives of an etch/acti- under potential control at all times, Rinsing is not included between vate electrolyte. The nickel strike keep the plated area 100% covered the process steps in Table 1 because, and buildup were plated using the by the wrapped anode, use rapid when the titanium surface is Future research work should continue to develop principles for good adhesion of plated deposits to titanium alloys acid nickel sulfate brush plating Table 2 Average tensile adhesion of nickel deposits on Ti-6Al-4V solution. Nickel plated readily into the microroughness features. Surface Type of Acid Nickel Solution adhesion, psi Adhesion was checked by tape Machined Sulfate 7,400 and bend tests, and the nickel deposit did not separate from the Blasted Sulfate 6,500 Ti-6Al-4V substrate. Thin panels of Blasted Sulfamate 6,600 Ti-6Al-4V and Ti-6Al-6V-2Sn broke in one 180° bend without separa- EC2086 epoxy 13,000 tion or peeling of the deposit. Grade 2 titanium can be bent a full 180° without breaking. The bend switching from anodic to cathodic, removed from the electrolyte, the test was more rigorous in revealing allow no rinsing between steps and oxide film readily forms in either low adhesion than the tape test. do not reuse the solution. air or water. This interferes with Several factors contribute to the Tests produced further under- adhesion between the deposit and excellent adhesion: mechanical standing of both the chemistry and the substrate. The solutions we used interlocking, increased surface mechanical processing needed in this work are compatible and a area and lack of an oxide film. to activate the titanium surface. water rinse between the process These three attributes were gen- Scale on the rolled panels had to steps is not required. A final rinse erated during the brush plating be removed mechanically, and removes electrolyte components process. Brush plating is particu- abrasion of panels was a require- after plating is complete. larly suited for generating these ment for good adhesion in the Scanning electron microscope attributes because of the small absence of blasting. Grit blasting, images of the titanium surface after volume of electrolyte, close contact wire wheel abrasion, grinding and each process step clearly show roll- between the anode and the cath- abrasive pad rubbing were effective ing lines in the as-received panels. ode, and the rapidity with which in preparing the surface for the Abrasion replaces the rolling lines with a new mechanical finish, and SiC grit-blasting, then abrading, the alloys, and this compromises the anodic etch further increases etching and activating using the adhesion. It is speculated the surface area. At high magnifica- process in Table 1. A 50-µm thick alloying constituents are more tion, micron- and sub-micron-size nickel deposit was plated from two prone to dissolution during the etch features are observed in the acid electrolytes. The failure mode anodic etch than titanium, lead- surface. These represent good in all specimens was adhesive, at the ing to the microrough surface anchoring points for subsequent nickel coating - titanium interface. morphology. deposits, and the subsequent nickel Of note, the grit-blasted surface did strike layer is dense with a slightly not increase the tensile adhesion Other Work nodular morphology. versus the machined surface. The We also developed a procedure bond strength did not depend for electroplating a duplex coating Surface Roughness on the type of acid nickel plating on Ti-6Al-4V. The substrate was We measured roughness changes solution used for the strike plate prepared by the process in Table produced by each surface treat- and buildup. 1, applying a 35-µm thick (0.0014 ment using a mechanical profi- Hydrogen embrittlement was in.) nickel layer. The nickel-plated lometer. Although the panels had tested according to General Motors Ti-6Al-4V was brush plated using a rolling direction, average rough- Engineering Standard GM3661P, an acid copper electrolyte to a ness was similar in both directions. in which a sustained test load in thickness of 350 µm (0.014 in.). Scotch-Brite abrasion decreased bending of 85% of the yield is This structure was then machined roughness slightly, but roughness applied to the part for 24 hr. Ti-6- to reduce the copper roughness. was increased by anodic etching. Al-4V coupons predrilled with a No delamination of copper from Microroughness induced by anodic 0.25-in. diameter hole for a stress nickel or nickel from Ti-6Al-4V was etching is a key component in riser were plated on one side observed because of machining attaining good deposit adhesion with 15 µm (0.0006 in.) of nickel stresses. The machined copper was to the Ti-6Al-4V panel The anodic using the process in Table 1. Six then brush plated with a 60-µm etch removes 0.0089 g/cm2 (0.0008 coupons were plated; three were thick (0.0024-in.) nickel cap. This in.) of titanium. tested as-plated and three were duplex coating structure is often The activation step designed to heat treated at 190°C (375°F) for used to repair surface defects. reduce the oxide formed during 24 hr prior to test. Future research work should con- anodic etching reduces the surface All samples were satisfactory for tinue to develop principles for good roughness. The average roughness hydrogen embrittlement to specifi- adhesion of plated deposits to tita- (Ra) in both directions after activa- cation GM3661P—that is, no failure nium alloys, and identify a process tion was somewhat lower than Ra or cracking was observed on the to deposit coatings with improved after etching. We determined that any of the coupons. The effect of adhesion on Grade 2 titanium. roughness of the plated layer was hydrogen embrittlement was thus Deposition of other materials with more a function of the nickel brush considered negligible. better wear resistance than titanium plating parameters. This technology also performed will also be investigated.n We also used a scanning electron well with Ti-6Al-6V-2Sn alloy, and For information from SIFCO Applied microscope to examine the inter- deposit adhesion was satisfactory. Surface Concepts, phone 216-524-0099 face between the deposit and tita- However, the procedure does not or go to www.sifcoasc.com. nium alloy substrate. Microrough- provide a deposit with adequate ness and the lack of a bulk oxide adhesion on Grade 2 titanium. LearnMORE film on the titanium was evident. Deposits on Grade 2 generally There was no sign of separation passed tape but failed bend tests. Avoid Nickel Plating Losses between the two metals. A variety of processes and chem- To learn how to minimize loss of nickel istries did not microroughen and metal in your plating operation, go to pfon- Other Evaluations activate Grade 2 titanium. line.com/articles/090503.html. Tensile adhesion of brush-plated The difference appears during nickel to Ti-6Al-4V was measured anodic etching. The alloys micror- following ASTM C 633. Average oughen, while a thin, visible oxide Reprinted from values measured on multiple film forms on Grade 2 and persists PRODUCTS FINISHING Magazine stubs are reported in Table 2. The through cathodic activation. Com- December 2009 and Copyright © 2009 by Gardner Publications, Inc., surface of the titanium alloy stubs mercially pure titanium appears 6915 Valley Ave., Cincinnati, was pretreated by machining or to passivate more quickly than Ohio 45244-3029