Electrocoat

Immersion and current, not spray, deposit the film in electrocoat. The metal part is immersed in a waterborne paint bath and made one electrode of a direct current circuit. Current passing through the bath electrolyses water at the surface of the part and shifts the pH there, so the charged resin and pigment that have migrated to the part lose their solubility and deposit as a coherent film. The part is then rinsed and cured in a bake oven. Like powder, electrocoat is an application technology rather than a market: the same tank finishes automotive components, off-highway machine parts, appliance chassis and general metal fabrication, and on most lines it is the primer beneath a powder or liquid topcoat rather than the visible finish. Arctic’s electrocoat range covers cathodic epoxy for corrosion priming, cathodic acrylic where the e-coat is also the finish, anodic materials for decorative work and components of moderate corrosion demand, and the feed materials and additives an operating tank consumes.

The defining behaviour is that deposition self-limits. As the film forms it insulates the surface, current through that area falls, and deposition continues wherever the film is still thin, so coating carries on into the inside of box sections, the back of flanges, blind holes and weld seams that a spray gun cannot see. That is the reason to choose the process. It is throwing power rather than perfect uniformity: build falls with distance into a cavity, so an interior surface finishes thinner than the outside, and the bath has to be able to enter and drain, which means closed cavities need access and drain holes if they are to be coated at all. Because the bath is a closed loop, with ultrafiltration and permeate rinses returning dragout to the tank, transfer efficiency is high by the nature of the process, and the material is waterborne, so solvent content is low compared with conventional liquid spray. The limits follow from the same physics. The substrate must conduct, and the part must be hung so that current reaches every surface. It must arrive clean and correctly pretreated, and the finished film owes as much to the pretreatment stages as to the coating itself. The film is thin by comparison with powder, so electrocoat is a corrosion primer or thin finish rather than a build coat. The tank also holds one colour at a time, so a line is committed to that colour until the bath is emptied and recharged, which is why most e-coat lines run a black or grey primer.

Electrocoat cannot be added to a spray booth, and the plant it requires is worth setting out in full. A line needs multi-stage pretreatment with cleaning, rinsing and a phosphate or zirconium conversion coating, with effluent treatment for those rinses; the e-coat tank itself with continuous circulation, filtration and temperature control; DC rectifiers feeding counter electrodes housed in their own cells, anolyte cells in a cathodic system and catholyte cells in an anodic one, the cell circuit removing the acid or amine released at the counter electrode so bath pH stays in control; ultrafiltration and permeate rinse stages; a demineralised water rinse; and a bake oven. Parts need jigs that hold them in reliable electrical contact and let the bath drain out of cavities. Industry practice is to cure conventional cathodic electrocoat in the region of 175 °C to 190 °C metal temperature, with lower bake grades formulated below that, and that figure describes general practice rather than any Arctic grade. The tank is a live chemical system that has to be circulated, monitored and replenished continuously rather than run in batches, and it deteriorates if it is left standing. For that reason e-coat suits high volume repeat production of similar parts. It does not suit job-shop work or short runs, and where the volumes do not justify a line, a powder or liquid system is the more sensible route. Electrocoat is a new capability area for Arctic Materials, supplied against the customer’s own bath control and analysis regime, with a Technical Data Sheet and a Safety Data Sheet for each grade.

Cathodic epoxy grades for corrosion priming of pretreated steel fabrications. These are the corrosion grades of the range, deposited at the cathode so no metal dissolves from the part into the film. Epoxy binders chalk and lose gloss under sunlight, so a CE grade is normally the primer beneath a liquid or powder topcoat, or the finish on parts that are not exposed.

Cathodic acrylic grades that hold colour and gloss on exterior exposure well enough for the electrocoat film to serve as the finish rather than only a primer. They are specified where a single dipped coat has to carry appearance, and they accept that the tank is committed to one colour for the duration of the run.

Anodic grades, pigmented and clear, in which the part is the anode. The bath is simpler and lower in cost than a cathodic system, and some metal dissolves from the part and is carried into the film, which limits corrosion performance. Specified for interior components of moderate corrosion demand and for decorative clear films over plated, polished and anodised surfaces.

Two routes for lines whose pretreatment or substrate mix does not suit a conventional zinc phosphate and cathodic epoxy sequence: an autodeposition coating that builds chemically on ferrous metal with no applied current and no conversion coating stage, and a cathodic electrocoat formulated to run over thin-film zirconium pretreatment on mixed-metal loads.

The figures below are the typical operating window for this range, not a measured result for any one batch. Confirmed values are issued with the Technical Data Sheet and Certificate of Analysis by our laboratory.

How the film is deposited

The part is the electrode. It is immersed in the waterborne bath and connected into a direct current circuit, negative in a cathodic system and positive in an anodic one. Current passing through the bath electrolyses water at the surface of the part and shifts the pH in the thin layer against the metal, so the charged resin and pigment that have migrated there lose their solubility and deposit as a coherent film. Continued current draws water out of that deposit by electro-osmosis, which is what makes the wet film firm enough to be rinsed before it is baked. The film that forms is insulating, so current through the coated area falls and deposition moves on to whatever surface is still bare. That self-limiting behaviour is the reason to use the process: build evens out on open faces and carries on into the inside of box sections, the back of flanges, blind holes and weld seams that no spray gun can see.

What throwing power actually means

Electrocoat reaches into recesses that spray cannot, and it is worth being precise about the limit. Current density falls with distance into a cavity, so film build falls with it. The interior of a box section finishes thinner than the outside, not equal to it, and the drop is measurable. Throw is a property of the bath as much as of the grade: conductivity, rupture voltage, applied voltage and coating time all move it, and an anodic bath normally throws less than a high throw cathodic epoxy. A high throw grade such as Arcdip CE 340 pushes that gradient further into the fabrication but does not remove it. The bath must also be able to enter and drain, so a sealed cavity stays uncoated whatever the grade, and access and drain holes belong in the fabrication drawing rather than in a later fix. Where an interior film thickness is contractual, it is agreed as a separate figure from the open surface figure and measured at named points.

The plant the process requires

Electrocoat cannot be added to a spray booth. A line needs multi-stage pretreatment with cleaning, rinsing and a phosphate or zirconium conversion coating, with effluent treatment for those rinses. It needs the coating tank itself under continuous circulation, filtration and temperature control, direct current rectifiers feeding counter electrodes housed in their own cells, and anolyte cells in a cathodic system or catholyte cells in an anodic one so the acid or alkali released at the counter electrode leaves the system and bath pH holds. It needs ultrafiltration with permeate rinse stages to recover dragout back into the tank, a demineralised water rinse, and a bake oven able to hold the metal temperature the grade requires. Parts need jigs that keep reliable electrical contact and let the bath drain. The tank is a live chemical system, replenished continuously against analysis rather than run in batches, and it deteriorates if left standing, which is why the process suits high volume repeat production and not short job-shop runs.

Primer or finish

Cathodic epoxy is the corrosion reference of the range and it chalks and loses gloss under prolonged sunlight, so it is normally a primer with a liquid or powder topcoat over it. That pairing is agreed before the primer is specified, because the primer bake, the surface condition it leaves and the recoat window all have to suit the coat that follows. Cathodic acrylic is the answer where a single dipped coat must hold exterior colour and gloss on its own. Anodic grades serve interior and decorative work where the lower cost bath is justified and the corrosion demand is moderate.

Bath control and supply

Composition drifts with throughput. Each electrocoat grade is therefore supplied as feed materials, resin emulsion and pigment paste, with the solubilising acid or amine the system runs on and correction additives including anti-crater and edge protection, against the line’s own control and analysis regime rather than as a one-off order. The tank holds one colour at a time, so the line is committed to that colour until the bath is emptied and recharged. Arcdip AU 120 is the exception in the range: it carries no current, and its bath is controlled on free acid, oxidiser level, redox potential and dissolved iron rather than on voltage and conductivity.

How to read the figures

Every value in the table above is a typical operating window for this class of process, published so that a plant can judge whether its line, its rectifiers and its oven are in the right region. None of them is a measured result for an Arctic grade. The salt spray, humidity and accelerated weathering figures are comparative laboratory results on prepared panels and are not a prediction of service life. Grade specific values, bath make-up instructions and control limits are given on the Technical Data Sheet, with a Safety Data Sheet for each grade and a compliance dossier issued on request.

Electrocoat is specified against the line as much as against the part. The following are the questions Arctic needs answered before a grade is proposed.

The part

• What is the substrate, and does one jig carry more than one metal? A steel only load keeps autodeposition open as a route; steel with zinc-coated steel and aluminium in the same load does not.
• Does the fabrication contain box sections, blind holes or closed cavities, and where are the access and drain holes? Can they be moved if the coating requires it?
• How does the part hang, and where does the jig make electrical contact? Contact marks land somewhere, and that position is chosen rather than accepted.
• Is the part exposed to sunlight and weather in service, or is it interior?

The requirement

• Is the electrocoat the finish, or a primer? If a primer, which topcoat follows, applied by which process, and what recoat window does the line allow between bake and topcoat?
• What film build is required on the open surface, and separately, what is the minimum acceptable build inside the cavity and at which measurement points?
• What is the service environment, stated where possible as a corrosivity category to ISO 12944-2, and what is the corrosion requirement stated as a test, a duration and a scribe assessment limit rather than as a general expectation?
• If exterior appearance is part of the requirement, what exposure test and duration is it to be agreed against, and what gloss and colour retention is acceptable at the end of it?
• What colour and gloss, and is the line prepared to commit the tank to that colour for the run?
• Is corrosion protection governing, which points to a cathodic grade, or is the work decorative with moderate corrosion demand, where anodic is viable?

The line

• What pretreatment is installed: zinc phosphate, iron phosphate or thin-film zirconium, and what coating weight and rinse conductivity does it hold? Is a change of pretreatment planned?
• What is the tank volume, the throughput in square metres per hour and the resulting bath turnover? Composition drifts with load.
• What rectifier capacity and voltage range is available, and how are the counter electrode cells arranged?
• Is ultrafiltration with permeate rinse fitted, and is there a final demineralised water rinse? What is the filtration rating and the temperature control range?
• What maximum metal temperature and dwell can the oven hold, and does any assembled component, fastener, bearing or seal limit it?
• Is the tank being charged from new, recharged, or converted from another supplier’s material while running?
• Who analyses solids, pH, conductivity, pigment to binder ratio and neutralising agent content, at what frequency, and against whose limits?

Documentation

• Which documents are required with delivery beyond the Technical Data Sheet and Safety Data Sheet, and is a compliance dossier needed for the end use? Dossiers and declarations are issued on request.

The Technical Data Sheet, Safety Data Sheet and regulatory declaration for any grade in this range are issued by our laboratory against the substrate and process you are running. Tell us the job and we will specify against it.