Aluminium blackening process aims to replace anodising
A coatings specialist has developed a scratch-resistant blackening finish for aluminium which it claimed is a faster and simpler alternative to black anodising.
The Lumiclad process takes 30 minutes and develops a 1.5 micron coating with a durable, clean surface that is electrically conductive and tightly adherent to the aluminium substrate, according to developer Birchwood Casey. The finish has a smooth crystal structure that absorbs an optional topcoat, such as a dry-to-touch sealant or clear polymer.
The company said that it is an ideal finish for precision machined components with critical dimensions, or for mating surfaces that require break-in lubricity and galling resistance. It added that it has a uniform thickness that will not close down hole diameters or change critical part dimensions, and its inherent lubricity makes it a good choice for sliding contacts, such as piston/cylinder assemblies, piston ring grooves, etc.
The Lumiclad process uses zinc/molybdenum chemistry - it starts by depositing a very thin layer of metallic zinc on the aluminium surface, which serves as a primer for the molybdenum black oxide finish. An immersion tank line capable of processing up to 680kg (1,500lb) of work per shift would require a series of seven 150 litre tanks and cost around £14,300 ($22,000), including the necessary chemical products and an ion exchange system from Siemens Water Technologies for waste treatment.
Conventional black anodising is a longer and more complicated electrolytic process, noted Mark Ruhland, vice president of Birchwood Casey. Parts must be manually clamped onto spring-clip racks to ensure electrical contact, which makes it hard to process small parts economically. While the anodised finish is extremely durable, coating can be thick enough to cause a measurable dimensional change and close down critical hole diameters, he said.
In contrast, Lumiclad requires no electrical current or clamping, so can be used for both large parts on racks and smaller parts in baskets or rotating polypro barrels. “The Lumiclad concept is a very significant finishing breakthrough,” said Ruhland. “No other non-electrolytic blackening process offers as many benefits to the user in terms of easy and safe operation, streamlined workflow, low operating costs and high quality.”