Anodizing dyeing

An assumed 8 µm outward growth per wall closes a 10.000 mm bore by 0.016 mm, giving a 9.984 mm finished bore. That misses the example's 9.990 mm minimum by 0.006 mm, even if the shade is correct. Confirm actual growth, pretreatment removal and masking, and define color acceptance and sealing separately.

Process and scope

Specify the alloy, porous anodic oxide, coloring route and seal as one finish system. Define shade and performance separately.

HAI describes organic immersion coloring as dye entering the pores and electrolytic coloring as metal salts introduced using electricity. These are distinct routes, even when both produce a nominally similar shade. Its service offering separates HAI Indoor and HAI Outdoor ranges and cautions that castings, plate and profiles can differ in appearance. HAI dyeing methods

This article concerns aluminum anodizing and coloring. It does not supply a bath formulation, electrical cycle or universal sealing recipe. Select the process for the actual alloy, geometry, appearance and service requirements.

Standard and named process example

ISO 7599:2018, edition 3, January 2018, confirmed in 2023, specifies decorative/protective anodic coatings and related properties/tests. Its exclusions include barrier-layer coatings, chromic/phosphoric-acid coatings, coatings solely preparing for later organic coating/electrodeposition, and engineering hard anodizing primarily for wear resistance. Do not use it as a universal specification for every process called anodizing. ISO 7599 scope

For a bounded customer-specification example, NASA/JSC PRC-5006 revision D, June 2020, distinguishes Type II sulfuric-acid anodizing from Type III hard anodic coatings, and Class 1 non-dyed from Class 2 dyed. Its Class 2 example includes the color in the drawing note. These labels belong to that specification and its referenced requirements; they do not replace a complete coating callout or establish compliance with ISO 7599. NASA process specification, printed pages 4–5

State the governing specification/revision, coating type, required thickness and tolerance, coloring route, color acceptance and sealing requirement. Separate appearance requirements from wear, corrosion or other performance requirements.

Why alloy and preparation affect color

The oxide is not an opaque paint film that necessarily hides the underlying metal and its texture. HAI describes a transparent layer whose visual appearance can be tailored through pretreatment and coloring. A change of material form or surface preparation can therefore matter even when the same dye is used. HAI coloring

The Aluminum Anodizers Council identifies alloy, temper, gloss, surface structure and coloring process as contributors to appearance, and recommends agreeing an acceptable range for each project. A single named color or screen image is not an adequate definition of metallic appearance. AAC coating specifications

As a concrete supplier practice, HAI offers trial runs and minimum/maximum color references made using the alloy intended for production. That is more useful than approving a sample made from an unrelated alloy or finish. Agree significant surfaces and viewing conditions with the supplier, particularly where adjacent parts must match. HAI samples and trial runs

Retain the material identity and preparation route with approved samples. Resolve mixed alloys, welds, cast material, machining marks and cosmetic defects before production. A sample approval should say what variation it permits; it should not silently waive unrelated thickness or performance requirements.

Sealing and service suitability

HAI explains that porous oxide can be sealed by a hydrothermal reaction, helping limit contamination and retain the color. It also notes that an unsealed condition may be selected for subsequent coating or bonding. Thus “always seal every anodized surface” is too broad: the finishing system and subsequent operations determine the requirement. HAI sealing

Sealing is not the same operation as dyeing, and a good initial shade does not establish seal quality. Identify the selected sealing process and applicable acceptance tests. Do not replace a specified sealing method merely because another method produces an acceptable visual result.

AAC's technical FAQ states that lightfastness depends on multiple factors, not just dye selection. Specify the relevant exposure and performance requirement rather than assuming every dyed coating is suitable for permanent outdoor service. AAC anodizing FAQ

HAI's inspection offering separately lists lightfastness, coating-thickness, color-drop and mass-loss tests. These test categories answer different questions. A color check does not establish corrosion resistance, and an accelerated exposure result is meaningful only with its method, duration, specimen and acceptance criterion. HAI test overview

Geometry, contacts and masking

Anodizing changes dimensions because oxide extends into and beyond the original surface. Growth is process-dependent. NASA's Type III guidance describes roughly half the coating thickness as outward growth, but cautions that internal and external thickness can differ on intricate geometry. Treat that proportion as an estimate for the identified process, not a machining guarantee. NASA revision D, printed pages 6–8

Electrical rack contact is necessary and leaves a local coating void. NASA directs coordination of important contact locations. Define permitted contact marks and protected significant surfaces before processing. NASA revision D, printed page 5

Identify bores, threads, sealing lands, electrical contacts and other features needing selective treatment or masking. Clarify the treatment at mask boundaries and any remaining protection required. Specify whether dimensions apply before or after finishing; where necessary, preserve both machining and finished limits. Do not assume a uniform coating inside a deep narrow feature merely because accessible outer surfaces meet thickness.

Original bounded dimensional calculation

Use the assumed 8 µm outward growth per wall. Ignore pretreatment removal, nonuniformity and measurement error:

bore diameter reduction = 2 × 8 µm = 16 µm = 0.016 mm

finished bore = 10.000 − 0.016 = 9.984 mm.

Against a specified 9.990–10.010 mm range, the result calls for an approved allowance, masking or another compatible route.

Handoff and acceptance

Preserve the part/revision, alloy/temper/form, surface preparation, governing coating specification, significant surfaces, thickness limits and locations, color range/samples, exposure requirements, seal, contact/masking map and finished dimensional requirements. Keep appearance, thickness, seal quality and performance results distinct.

Use representative parts where geometry or material variation may affect the outcome. Resolve rework before stripping or recoating: a repeat process must still meet the finished dimensions and all specified properties, not just restore color. These are proposed handoff questions rather than a mandatory report template.

Source versions

ISO's record provides the edition and confirmation dates above. NASA's actual PDF cover identifies PRC-5006 Rev. D, June 2020; its approval page is dated June 22, 2020. No date is inferred from the download path. HAI and AAC live guidance has no verified technical revision date here; their copyright years are not issue dates. AAC content was retrieved through primary indexed search text where direct access was restricted. The dimensional example is original arithmetic.

Surface-finishing processes