Reprinted from Finishing - September/October 2016 © Turret Group 2016 • www.finishingmagazine.co.uk

Mention “brush plating”and it conjures upimages of a primitive

process using plating solutiontaken from a tank and appliedusing a powerpack attached toa swab. This is an outdatednotion of the process. Withmodern techniques, selectiveelectroplating on a small scalecan be a key weapon in thesurface treatment armoury.Small scale electroplating

allows sections of componentsto be treated withoutimmersion and with minimalmasking. That means thatthreads and inserts can beplated, hanger witness markseliminated and all manner ofrepairs accomplished to anoriginal treatment both in oemand maintenance environments.Equipment which has beenassembled and containssensitive components can betreated without risk. In additionselective electroplating can beused to replace metal and buildup dimensions on scratchesand inside bores and otherareas where there is insufficientmaterial, due to over-machiningor in service wear.Practically any deposition can

be applied using this methodincluding anodising and metalplating. Some characteristics

can even be improved upon relative to in-tank plating as the high current densityachieves a faster result and the coolertemperatures reduce stress. Essentially a smallanode is used to apply the plating directly tothe object using a wet textile reservoir of theplating solution. The composition of thesolution and the voltage and current densityare closely controlled to ensure that theplating layer is developed exactly as required.Dalic, the pioneer and leading supplier of

selective electroplating solutions is leading theway in developing new technologies both inthe chemistry of the materials and in thedesign of application equipment. Manyimportant aerospace companies haveapproved Dalic brush plating includingAirbus, BAE Systems, Boeing, Agusta-Westland, Dassault, Embraer, SAFRAN andUTC. The operators of this equipmentincluding airlines, MROs and defenceorganisations are able to use the sameprocesses for maintenance.

Cadmium and ZinidalCadmium plating is still frequently applied

in the aerospace and defence sectors, this isboth for new parts which have beendamaged in production and for repair of in-service parts which have suffered damagethrough operations. The Dalic Cadmium2023 process is widely used in the industryand each batch of 11028 Cadmium platingsolution is tested LHE to ASTM F.519 forhydrogen embrittlement and certified beforeshipment. On high tensile steels a pre-treatment is not needed but many processesrequire pre-plating with nickel to give optimaladhesion. Using a rectifier with a graphiteanode attached, covered in a fabric bonnetand supplied with 20V and a controlledcurrent, it is possible to deposit up to6.4μ/min when the contact area is 1/3 of thearea to be plated.Cadmium is a harmful substance and there

is a high level of interest from industry inreducing the amount of Cadmium used andin qualifying viable alternatives. Zinc Nickel(Zi-Ni) is a very high performing platingwhich is being qualified in the aerospace

Brush plating for the21st century

CASE STUDY

Process Substance Measured Maximum authorised

Cadmium plating Cadmium 0.0001 mg/m3 0.05 mg/m3

CAA Anodising Chromic Acid 0.0005 mg/m3 0.05 mg/m3

Zinc-Nickel plating Nickel sulphate 0.0008 mg/m3 0.1 mg/m3

Reprinted from Finishing - September/October 2016 © Turret Group 2016 • www.finishingmagazine.co.uk

CASE STUDY

industry as an alternative to Cadmium. Dalic’sZinidal is a class leading process for localisedapplication and satisfies EN2486 (adapted fortouch-up application) on steels and aluminiumalloys, including corrosion, thickness ofdeposition and adhesion.

Other metal platingPractically every form of metal plating can

be applied using the small scale selectiveplating approach. Dalic solutions are designedto drop into the process with minimal pre-treatment and clearly defined electricalrequirements provided in the product dataand through regular on-site training courses.Some very interesting localised plating

solutions are possible. For example, gold andsilver plating for both technical (such aselectrical conductivity) and aesthetic purposes.Dalic gold plating produces a layer of 99.9%pure gold and with a simple 6V powerpack alayer of up to 3.2μ/min can be built up oversmall areas.Other metals including copper, tin, cobalt,

nickel, indium and zinc can be applied. Dalic’szinc plating solution is approved andrecommended by Peugeot at its repair centresto restore the original galvanising and ensurethat the corrosion warranty is maintained. Thissimple local zinc plating set-up is suitable forrepairs to original zinc plating on all types ofvehicle and installed equipment.Selective nickel plating is an excellent way

to resize worn or over-machined mechanicalparts or to improve electrical conductivity,many major engineering companies use DalicNickel for these activities including Thales,MBDA, ,MAN, Alstom and GE Power. It canalso be used for small repairs to electrolessnickel plating in the oem situation, thisincludes the removal of witness marks whereparts have been suspended by wires toimmerse them in the treatment tanks. Partscan also be repaired further along the

production line after assembly.

AnodisingThe selective plating process can be

extended to anodising and an excellent qualityanodised layer can be realised for both CAA,SAA and PAA (chromic, sulphuric andphosphoric acid anodising). A higher voltage isrequired and the process is improved by theuse of a cooling system and recirculation ofthe solution. The layer thickness can be from1µ for PAA before structural bonding up to12µ for CAA & SAA anodising to preventcorrosion and up to 100µ for CAA or SAAhard anodising to restore tolerance of a boreor to improve wear resistance. The Panta gelprocess allows anodising on larger surfaceswithout any additional equipment and this isoften used in the restoration of aerospacetreatments in the field when bonding or otherrepairs are carried out.Chromium trioxide and sodium dichromate

are both identified as SVHCs under REACHand Dalic is a member of the CTACconsortium applying for authorisation for theircontinued use. If the substances are authorised

then Dalic’s downstream users will bepermitted to carry out selective CAA withinthe authorisation. Most aerospace manufacturers including

Airbus are seeking chromate free alternativesto CAA. Both SAA and TSA (tartaric sulphuricanodising) are approved to variousspecifications. Dalic’s SAA solution is wellproven and widely used, TSA is now alsoavailable and work is in progress to fine tunethe colour of the TSA to provide an invisiblerepair to the original treatment.

Dalistick stationA combination of factors drove Dalic to

design and patent the Dalistick station. Wherehazardous metals such as cadmium, chromeand nickel are used, then the work should becarried out in an area with adequate airextraction. This reduces the concentration ofmetal in the air but is not always feasible in areal life situation.For anodising processes both cooling and

recirculation of the solutions is advantageous.The Dalistick station is a mobile workstation

containing a high voltage rectifier, recirculationpumps and a cooler to deliver a self-containedelectroplating “mini factory” right where it isneeded. This is combined with the Dalistick,which is an application tool with suctionapplied at the surface. The suction is greaterthan the feed and all the plating solution isreturned to the reservoir without any drips ordroplets forming in the air. This results in avery safe and clean application environment. Tests carried out using “on worker”

sampling by the French government’s APAVEagency showed very low amounts of metal,well below the exposure limits over a 2 hourapplication period without any extraction.Selective “brush” electroplating is now a

high tech solution which can form a vital partof an efficient surface treatment portfolio,reducing process time and permitting localisedapplication without expensive infrastructure.

Pexa T. +44 (0) 1422 314400F. +44 (0) 1422 314401www.pexa.com