Which anodizing defects make a lot rejectable, and how do you trace anodizing defects back to alloy, machining or the anodizer?

What counts as an anodizing defect, and which ones make a lot rejectable?

A defect is rejectable when it breaks a requirement you can point to: the workmanship clause of the cited specification, a contact-mark location on the drawing, or a cosmetic standard named on the purchase order. Powdery or loose film, breaks in the coating and damage that reduces serviceability fail the specification on their own; most purely cosmetic marks fail only if your documents say so.

The workmanship paragraph of the military specification requires the anodic coating to be continuous, smooth, adherent and uniform in appearance, and free from powdery areas, loose films, breaks, scratches and other defects that would reduce the serviceability of the parts (MIL-PRF-8625F, paragraph 3.13). The same paragraph says that appearance differences caused by the base metal itself, such as welds, a component with both cast and machined surfaces, or grain-size differences within a forging, are not cause to reject the coating unless the contract or purchase order says otherwise, and it allows slight discoloration where sealing solution drips or runs out of designed crevices. Contact marks are expected: paragraph 3.13.1 only asks that their size and number be the minimum consistent with good practice, and fixes their location only if the drawing, contract or purchase order specifies one.

That leaves a practical split for anyone writing a rejection. Functional defects, meaning soft or powdery film, coating breaks, burns that have eroded the metal, pits on a sealing face, crazing on a part that must resist corrosion, and bare areas where coating is required, are rejectable against the specification itself. Cosmetic defects, meaning streaks, mottling, a rack mark on a visible face, faint water marks or a shade difference, are rejectable only against a cosmetic standard you supplied: limit samples, marked significant surfaces, or written viewing conditions. If none of those exist, a cosmetic rejection is an opinion, and the anodizer will usually win the argument.

Lot acceptance under the specification is also narrower than many buyers assume. Conformance inspection consists of a visual examination, a color examination for dyed coatings and a thickness check for hard coat, sampled from each lot under MIL-STD-1916 at Verification Level II, with the acceptance criteria to be set in the contract or order (paragraph 4.4). A lot is one part number run in the same tank, by the same process, type and class, and no larger than one shift’s output. If your purchase order does not set them, there is no agreed accept number to reject against.

How do you diagnose an anodizing defect before you reject the lot?

Map where the defect sits on the part and across the lot, then run a few simple checks: a rub test, a thickness reading, a look under magnification and, if staining is the complaint, a seal test. The pattern usually tells you whether the cause is the metal, the machining or the tank before anyone strips a single part.

Work through this checklist on a handful of rejected parts and a handful of good ones from the same lot:

• Where on the part? Edges, corners and thin fins point to burning. Pale or thin areas on faces that hung downward, or inside recesses and blind holes, point to trapped gas or solution. A mark in the same place on every part points to the rack position. Fingerprint-shaped or randomly scattered spots on flat faces point to contamination before or during anodizing. Lines that follow the extrusion or rolling direction point to the metal.
• Where in the lot? Ask the anodizer which parts ran on which rack and load. If the defect sits on one rack, one load or one shift, it is a process event. If it follows one bar heat lot or one casting date code, check the material certificates and trace the parts back to their stock.
• Rub test. Rub the area firmly with a clean white cloth. If a hazy or iridescent layer comes off and leaves a clean, glossy film behind, it is seal smut, a sealing-line issue that can often be cleaned off. If the surface keeps chalking however long you rub, the film itself is soft or powdery, which fails the workmanship clause.
• Thickness. Measure film thickness on the defect and on a sound area of the same face with an eddy-current gauge using the method in ASTM B244, keeping the probe off edges and tight radii. A burn usually reads little or no sound film at the spot; a thin reading across a whole face or a whole load points to the anodizing time, the current or the rack contact.
• Magnification. Under a loupe or low-power microscope, check whether there is anodic film inside each pit. Pits with no film over them, often dark and star shaped, formed during anodizing, typically from chloride in the electrolyte; shallow white pits sealed over by film were already in the surface before anodizing, from rinse water, handling or storage. Crazing shows as a fine spider-web network in the top of the film; cracks that run down to bright metal at sharp corners are more serious.
• Seal check. If the complaint is staining, fingerprints that will not wipe off or poor corrosion results, ask for a dye-stain test of the seal per ASTM B136, or the acid-dissolution weight-loss test per ASTM B680.
• History before the tank. Ask how long the parts waited between machining and anodizing, whether they were stored wet or in coolant, whether they were handled with bare hands or packed in damp paper or cardboard, whether steel brushes or steel wool were used for deburring, and whether different alloys or tempers were mixed in the lot.
• History after the tank. For crazing, ask whether the parts were baked, cured, hot-washed, bent or press-fitted after anodizing, and at what temperature.

Strip one sample part only when the checks above disagree. With the film removed, a pit or void that is already in the metal belongs to the material or casting, while a clean surface under the defect points back to the process. The process and handling entries in the Qualanod anodizing defects catalog, with photographs, are a useful reference when the anodizer and your inspector name the same defect differently. Keep the stripped part, the readings and photos together; they become the evidence in the supplier’s 8D report.

What should the drawing, PO and RFQ say so anodizing defects do not recur?

Name the specification with type, class and seal, mark the significant surfaces and the zones where contact marks are allowed, and write the visual acceptance criteria and sampling plan onto the purchase order. Then fix the upstream causes on the drawing: corner radii, alloy and temper, no mixed alloys in a lot, and a note on cleaning and storage before anodizing.

A workable set of notes, to adapt rather than copy:

• Specification. “Anodize per MIL-PRF-8625, Type II, Class 2, black, sealed” or the hard-coat equivalent with its thickness; the MIL-A-8625 types guide explains each designation.
• Significant surfaces. Mark faces “A” (cosmetic, visible on the product) and leave the rest functional. Without this, every face is judged the same.
• Contact-mark zones. “Rack contact permitted only in zone C” or on a named hole or flat. The specification lets you fix the location; it does not do it for you.
• Viewing conditions. State the viewing distance, the light source and how long the inspector may look at each face, chosen to match how the end user sees the part, and name the defects that are never acceptable on significant surfaces (burns, pits, bare spots, powdery film).
• Sampling. Name the sampling plan and accept number on the purchase order, as the AQL sampling guide describes, so lot acceptance is not decided part by part.
• Geometry. Break sharp edges with a radius on hard-coated parts, avoid blind holes and crevices that trap solution where you can, and anodize components before press-fit or welded assembly; the masking guide covers bores and threads that must stay bare.
• Material and handling. One alloy and temper per lot, no iron-bearing abrasives for deburring, parts cleaned and dried after machining, handled with gloves, and not stored wet or packed in damp paper before anodizing.
• After-anodize heat. If the part will be baked, cured, hot-washed or run hot, say so on the RFQ so the film thickness and seal can be chosen for it, and do any bending or forming before anodizing, as the specification already requires.

For castings, keep expectations in line with the metal. The specification accepts dye bleed-out and some lack of color uniformity on cast alloys, and MIL-PRF-8625F paragraph 6.16 notes that a hard coat of 2 mils (about 50 µm) or more is extremely difficult to obtain on high-silicon die castings such as 360, 380 and 383; the die-cast anodizing guide covers which alloys anodize acceptably.