Anodizing appears on drawings as a single word far more often than it should. “Anodize black” tells a finisher very little, and the gap between what the designer imagined and what arrives can be substantial.<\/p>\n\n\n\n
The main distinction is between Type II, the decorative and general-purpose process, and Type III, hard anodizing for wear resistance. They produce visibly and functionally different results, change part dimensions by different amounts, and differ considerably in cost.<\/p>\n\n\n\n
This guide covers what each does, how to choose, and what a complete specification includes.<\/p>\n\n\n\n
What Anodizing Actually Is<\/strong><\/p>\n\n\n\n Anodizing electrochemically grows an oxide layer on aluminium. Unlike plating or painting, the coating is not applied to the surface, it is converted from it.<\/p>\n\n\n\n That distinction matters practically. The oxide layer grows both outward from and inward into the original surface, roughly half in each direction. So a part gains dimension, but by less than the total coating thickness.<\/p>\n\n\n\n The resulting layer is porous as formed, which is what allows dye to be absorbed for colour. It is then sealed to close the pores, which is what provides corrosion resistance.<\/p>\n\n\n\n The coating is integral to the part and will not chip or peel the way paint can, though it is harder and more brittle than the aluminium beneath it.<\/p>\n\n\n\n Type II: Conventional Anodizing<\/strong><\/p>\n\n\n\n The general-purpose process, sometimes called decorative or sulphuric acid anodizing.<\/p>\n\n\n\n Coating thickness<\/strong> is typically in the range of roughly 5 to 25 microns, varying with the specification and application.<\/p>\n\n\n\n Appearance.<\/strong> Type II accepts dye readily and produces a wide range of colours. Black, blue, red, gold and clear are all standard. Colour consistency is good on suitable alloys, and the finish can be attractive.<\/p>\n\n\n\n Properties.<\/strong> Provides good corrosion resistance and moderate wear resistance, along with electrical insulation. Adequate for the majority of consumer, industrial and enclosure applications.<\/p>\n\n\n\n Dimensional change<\/strong> is small, typically a few microns per surface, which is negligible for most parts but not for precision fits.<\/p>\n\n\n\n \u0627\u0644\u062a\u0643\u0644\u0641\u0629<\/strong> is the lower of the two.<\/p>\n\n\n\n Typical applications:<\/strong> consumer product housings, enclosures, brackets, decorative components, general industrial parts, anything where appearance and corrosion resistance matter more than wear.<\/p>\n\n\n\n Type III: Hard Anodizing<\/strong><\/p>\n\n\n\n Also called hardcoat or hard anodizing, produced at lower temperatures and higher current density.<\/p>\n\n\n\n Coating thickness<\/strong> is typically in the range of roughly 25 to 50 microns, and can go thicker for specific applications.<\/p>\n\n\n\n Appearance.<\/strong> Naturally produces a dark grey to bronze or near-black finish depending on the alloy. Colour options are limited, and dyeing is possible but produces darker, less predictable results than Type II. If a specific colour is required, Type III is usually the wrong choice.<\/p>\n\n\n\n Properties.<\/strong> Substantially harder and more wear resistant than Type II, with better abrasion resistance and good corrosion protection. This is the reason to specify it.<\/p>\n\n\n\n Dimensional change<\/strong> is significant and must be accounted for in design. With roughly half the coating growing outward, a thick hardcoat adds meaningfully to external dimensions and reduces internal ones. On threaded features, bores and sliding fits, this matters and is a common source of parts that will not assemble.<\/p>\n\n\n\n \u0627\u0644\u062a\u0643\u0644\u0641\u0629<\/strong> is higher, both from the process itself and from the masking often required to protect features that cannot tolerate the dimensional change.<\/p>\n\n\n\n Typical applications:<\/strong> wear surfaces, sliding components, hydraulic and pneumatic parts, aerospace components, tooling, parts subject to abrasion.<\/p>\n\n\n\n Comparison<\/strong><\/p>\n\n\n\n Choosing Between Them<\/strong><\/p>\n\n\n\n Choose Type II when<\/strong> appearance is a requirement, a specific colour is needed, wear is not a primary concern, the part has precision fits that cannot tolerate dimensional change, or cost matters and the application is undemanding.<\/p>\n\n\n\n Choose Type III when<\/strong> the surface will experience abrasion or sliding contact, the part operates in a demanding environment, or hardness is a functional requirement rather than a preference.<\/p>\n\n\n\n The most common specification error is choosing Type III defensively for parts that will never wear, which adds cost, restricts colour options, and introduces dimensional changes that then cause assembly problems.<\/p>\n\n\n\n Designing for Anodizing<\/strong><\/p>\n\n\n\n Account for Dimensional Change<\/strong><\/p>\n\n\n\n The critical point, and the one most often missed.<\/p>\n\n\n\n Because roughly half the coating grows outward, external dimensions increase and internal dimensions decrease. For Type II this is usually negligible. For Type III it frequently is not.<\/p>\n\n\n\n Threaded holes are the common failure. A tapped hole anodized with a thick hardcoat may no longer accept its fastener. Either mask threads, or size them accounting for the coating, or specify Type II for threaded features.<\/p>\n\n\n\n Sliding fits, bearing bores and any dimension with a tight tolerance need the same consideration. Decide at design stage whether to compensate in the machined dimension or to mask the feature.<\/p>\n\n\n\n Masking<\/strong><\/p>\n\n\n\n Features that must not be coated require masking, which adds cost and needs specifying.<\/p>\n\n\n\n Common masked features include threaded holes, electrical grounding points, bearing surfaces requiring bare metal contact, and datum surfaces used for inspection.<\/p>\n\n\n\n Masking is done manually in most cases, so complex masking on a high-volume part becomes expensive. Where possible, design so that masking is simple or unnecessary.<\/p>\n\n\n\n \u0627\u0644\u062a\u0648\u0635\u064a\u0644 \u0627\u0644\u0643\u0647\u0631\u0628\u0627\u0626\u064a<\/strong><\/p>\n\n\n\n Anodizing is an insulator. Where a part requires electrical grounding or continuity, the contact area must be masked or the coating removed afterward.<\/p>\n\n\n\n This catches out designers of electronic enclosures regularly, where an anodized chassis is assumed to provide grounding and does not.<\/p>\n\n\n\n Alloy Affects Appearance<\/strong><\/p>\n\n\n\n Not all aluminium alloys anodize identically.<\/p>\n\n\n\n 6061 anodizes consistently and is a reliable choice for cosmetic parts. 7075 anodizes but the zinc content can affect appearance and batch consistency. Alloys with high copper content, such as 2024, produce darker results. Cast alloys generally anodize less predictably than wrought.<\/p>\n\n\n\n If colour consistency matters across a production run, specify the alloy and confirm the finisher has experience with it. Our comparison of aluminium grades<\/a> covers the alloy differences in more detail.<\/p>\n\n\n\n Surface Preparation Shows Through<\/strong><\/p>\n\n\n\n Anodizing does not hide surface defects, and in some respects it emphasises them. Machining marks, scratches and handling damage all remain visible.<\/p>\n\n\n\n Where appearance matters, specify the pre-anodize surface finish. Bead blasting before anodizing produces a uniform matte appearance that hides minor machining marks and is a common approach for cosmetic parts. Polishing before anodizing produces a bright finish.<\/p>\n\n\n\n The finish you specify before anodizing largely determines the finish you get after it.<\/p>\n\n\n\n Writing a Complete Specification<\/strong><\/p>\n\n\n\n A useful anodizing callout includes:<\/p>\n\n\n\n MIL-A-8625 is the specification most commonly referenced for anodizing types and classes, and citing it removes ambiguity.<\/p>\n\n\n\n Where colour matters, an approved sample is worth more than a colour name. Black from one finisher differs from black from another, and dye lots vary. Agreeing a physical reference at the start prevents disputes later.<\/p>\n\n\n\n Cost Considerations<\/strong><\/p>\n\n\n\n Type III costs more than Type II<\/strong>, both in processing and often in masking.<\/p>\n\n\n\n Masking adds cost proportional to complexity.<\/strong> Simple masking is inexpensive, complex masking on many features is not.<\/p>\n\n\n\n Tight colour matching adds cost<\/strong> through additional control and potential rework.<\/p>\n\n\n\n Rack marks<\/strong> are unavoidable. Parts must be held during processing, and the contact point does not coat. Specify where rack marks are acceptable, or the finisher will choose, possibly on a visible surface.<\/p>\n\n\n\n Batch size affects cost<\/strong> since anodizing is done in tanks, and small batches carry proportionally more setup.<\/p>\n\n\n\n Other Finish Options<\/strong><\/p>\n\n\n\n Anodizing is not always the answer.<\/p>\n\n\n\n Chemical conversion coating<\/strong>, sometimes called chromate or alodine, provides corrosion resistance with electrical conductivity and negligible dimensional change. Useful where grounding is required.<\/p>\n\n\n\n \u0637\u0644\u0627\u0621 \u0627\u0644\u0645\u0633\u062d\u0648\u0642<\/strong> provides a wide colour range and thicker protection, though it is an applied coating rather than a converted surface.<\/p>\n\n\n\n \u0627\u0644\u0637\u0644\u0627\u0621<\/strong> offers colour flexibility for parts where durability requirements are moderate.<\/p>\n\n\n\n \u0627\u0644\u062a\u0635\u0641\u064a\u062d<\/strong> provides different properties again, including harder or more conductive surfaces.<\/p>\n\n\n\n The right choice follows from what the surface must do. Our overview of surface finishing options<\/a> covers the alternatives.<\/p>\n\n\n\n\u0627\u0644\u0639\u0627\u0645\u0644<\/strong><\/td> \u0627\u0644\u0646\u0648\u0639 \u0627\u0644\u062b\u0627\u0646\u064a<\/strong><\/td> \u0627\u0644\u0646\u0648\u0639 \u0627\u0644\u062b\u0627\u0644\u062b<\/strong><\/td><\/tr> Typical thickness<\/td> Roughly 5 to 25 microns<\/td> Roughly 25 to 50 microns<\/td><\/tr> \u0627\u0644\u0635\u0644\u0627\u0628\u0629<\/td> \u0645\u0639\u062a\u062f\u0644<\/td> \u0639\u0627\u0644\u064a\u0629<\/td><\/tr> \u0645\u0642\u0627\u0648\u0645\u0629 \u0627\u0644\u062a\u0622\u0643\u0644<\/td> \u0645\u0639\u062a\u062f\u0644<\/td> \u0645\u0645\u062a\u0627\u0632<\/td><\/tr> \u0645\u0642\u0627\u0648\u0645\u0629 \u0627\u0644\u062a\u0622\u0643\u0644<\/td> \u062c\u064a\u062f<\/td> \u062c\u064a\u062f<\/td><\/tr> Colour options<\/td> Wide range<\/td> Limited, dark tones<\/td><\/tr> Dimensional change<\/td> \u0635\u063a\u064a\u0631\u0629<\/td> Significant<\/td><\/tr> Relative cost<\/td> \u0623\u0642\u0644<\/td> \u0623\u0639\u0644\u0649<\/td><\/tr> Primary purpose<\/td> Appearance and corrosion<\/td> \u0645\u0642\u0627\u0648\u0645\u0629 \u0627\u0644\u062a\u0622\u0643\u0644<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n \n