Guide
What causes aluminum corrosion on machined parts, and which alloy and finish callout prevents it?
Aluminum corrosion is the breakdown of the thin, self-healing oxide film that normally protects aluminum, which shows up on machined parts as a white, powdery film or crust, as dull stains, or as small pits under a white crust. It almost always traces back to water that stayed on the part, chlorides in that water, a copper-bearing alloy left bare, a damaged or unsealed finish, or a crevice or dissimilar-metal joint, and each of those causes has its own fix on the drawing.
By Saola Manufacturing engineering team · Published · Updated
This guide is for design, quality and sourcing engineers who opened a box of aluminum parts and found white powder, or who got a field return with pits around a fastener, and need to know what happened and what to change. It works from the symptom to the cause, gives a diagnostic table you can use at incoming inspection, and ends with a decision table that pairs the service environment with an alloy and a finish callout. Galvanic couples and finish selection each have their own guides, linked where they come up, so this page stays on the corrosion problem itself.
On this page
What does aluminum corrosion look like on a machined part, and is white powder always corrosion?
Real aluminum corrosion leaves a white or gray, chalky corrosion product that sticks to the surface and usually hides a pit, a stain or an etched patch underneath. White powder that wipes off cleanly and leaves bright, smooth metal is more often dried coolant, cleaning residue or a surface bloom than corrosion.
Aluminum does not rust the way steel does. The Aluminum Association’s guidance on water staining describes how aluminum combines with oxygen in air to form a tough, transparent, protective oxide, and how, in the presence of moisture, it stains rather than rusts (Aluminum Association TR3). That is why the first sign is cosmetic: a dull, irregular patch, a white bloom on an edge, or a ring where a droplet dried. The damage becomes functional when the attack goes local, as pitting under a white cap, as attack along grain boundaries in high-strength alloys, or as concentrated loss around a stainless or brass fastener.
Three forms cover almost every complaint about machined aluminum parts:
- Staining and general etching. Broad, shallow, often patterned like the contact area between two parts or a droplet outline. Mostly cosmetic, but it can spoil a later anodize or paint.
- Pitting. Small, scattered spots with a white or gray crust; brushing the crust off reveals a cavity. Pitting is the form that matters for sealing faces, thin walls and fatigue-loaded features.
- Localized attack at joints. Corrosion concentrated in a crevice, under a washer or gasket, or around a dissimilar-metal fastener. The galvanic corrosion guide covers the fastener case in depth.
What causes aluminum parts to corrode, ranked from most to least common?
Trapped moisture during storage and shipping comes first, chlorides that turn that moisture aggressive second, and a bare copper-bearing alloy third. Damaged or unsealed finishes, crevices and galvanic couples explain most of the rest.
- Water that could not dry off. The Aluminum Association states that virtually all staining problems occur during shipping, handling or storage, and that the stain forms when water is trapped between wraps, sheets or other mating surfaces with no air flow to remove it (TR3). Machined parts stacked face to face in a bag, or nested in a tray after a water-based wash, are exactly that geometry.
- Chlorides. Pitting in aluminum is most often caused by halide ions, especially chloride (Total Materia, Corrosion of Aluminum and Aluminum Alloys). On parts, chlorides come from coastal air, road salt, contaminated or tired coolant, rinse water, and bare-hand handling, since sweat carries salt.
- A copper-bearing alloy left bare. The 2xxx alloys, which use copper as the main alloying element, are less resistant to corrosion, and copper-containing 7xxx alloys such as 7075 are more susceptible as well, while 5xxx and 6xxx alloys are highly or excellently resistant (Total Materia). Free-machining 2011 and high-strength 2024 and 7075 therefore pit and stain sooner than 6061 in the same box.
- A breached or unsealed finish. Holes drilled or tapped after anodizing, masked areas left bare, handling scratches, and anodize that was never sealed all leave small bare anodes or open pores where moisture collects.
- Crevices and dissimilar metals. Lap joints, blind holes that face up, gaskets and washers hold water; stainless, brass, copper or carbon fiber in contact with the aluminum make it the anode.
High-strength tempers add one more failure mode that is not visible until it is serious. A NASA Marshall Space Flight Center memo found the high-strength aluminum alloys it tested highly susceptible to stress corrosion cracking in the short transverse direction, with 7075-T73 among the exceptions, and found that neither a chemical conversion coating nor an anodic coating on its own was effective against stress corrosion (Humphries, NASA TM X-50924). That is why the temper on the drawing matters more than the finish for a stressed 7075 part in salt air: the overaged T73-type tempers trade some strength for stress-corrosion resistance, and a coating does not make up for a T6 temper.
| Alloy | Copper, % by mass | Bare-metal behavior in damp or salty service |
|---|---|---|
| 2011 (free machining) | 5.0 to 6.0 | Poor; stains and pits quickly, nearly always finished |
| 2024 | 3.8 to 4.9 | Poor; prone to pitting and intergranular attack |
| 7075 | 1.2 to 2.0 | Fair to poor; T6 temper sensitive to stress corrosion and exfoliation, T73 more resistant |
| 6061 | 0.15 to 0.40 | Good; the default for parts that see weather |
How do you diagnose white powder and pits on an aluminum part?
Wipe it, look under it, and map where it is: residue wipes off and leaves sound metal, pitting leaves cavities, and attack that follows a joint, fastener or scratch points to a crevice, a galvanic couple or a finish breach. Pits can then be rated by density, size and depth per ASTM G46.
Work through the checks in order on a few affected parts and one clean part from the same lot. Wipe a spot with a clean, dry cloth, then with isopropyl alcohol. Look at the wiped area under a loupe or a stereo microscope with low-angle light. Note where the powder sits: on all faces, only on faces that touched another part or the bag, only at edges and holes, only near fasteners, or only where the finish was cut. Ask for the alloy and temper from the material certificate, the finish callout and whether it was sealed, and the storage and transit history. ASTM G46 is the guide for examining and evaluating pitting corrosion, and naming it gives both sides the same vocabulary for density, size and depth when a lot is disputed. MIL-A-8625F defines a pit as localized corrosion deeper than it is wide, which is a quick test under the loupe: a stain has no depth, a pit does.
If the parts arrived in a carton, add two receiving checks from the Aluminum Association’s guidance: note any wet or puckered wrapping on the receiving papers, and measure the metal temperature with a probe or contact thermometer and compare it with the dew point of the receiving area. Metal colder than that dew point condenses water on arrival, which points to transit and receiving rather than to the supplier’s finish (TR3, sections 4 and 5).
| What you see | Most likely cause | What to check next |
|---|---|---|
| Powder wipes off; metal underneath bright and smooth | Dried coolant, wash or rinse residue | Wash and rinse process, final rinse water quality, drying |
| Irregular stains where parts touched each other or the bag | Water staining from condensation or trapped water | Packing, desiccant, transit temperature swings, wet cartons |
| White crust; cavities after brushing | Chloride pitting | Chloride sources, alloy and temper, finish present and sealed |
| Pits only on bare holes, threads or machined spots | Finish breach (machined after finishing, or unmasked) | Operation sequence, masking notes on the drawing |
| Powdery bloom evenly over an anodized surface | Sealing residue, or unsealed or poorly sealed anodize | Seal requirement in the callout; a seal-quality test from the finisher |
| Attack concentrated around stainless or brass hardware | Galvanic corrosion | Fastener material, isolation, sealant, water path |
| Layers lifting along edges of 7075 or 2024 | Exfoliation or intergranular attack | Temper (T6 vs T73 or T7351), grain direction, metallography |
| Thread-like trails under powder coat from an edge or scratch | Filiform corrosion under the coating | Pretreatment under the coating, edge coverage, chlorides |
On anodized parts, MIL-A-8625F requires the coating to be free from powdery areas, so a powdery anodized surface is a workmanship nonconformance against that specification, not only a cosmetic complaint. If the cause is still unclear after this table, a metallurgical lab can section a pit and tell residue from corrosion product and pitting from intergranular attack. That is worth the cost when a lot is about to be rejected or a field failure involves a stressed part.
Which alloy and finish callout prevents aluminum corrosion in each environment?
Match the finish to the environment first and the alloy second: dry indoor parts in 6061 often need nothing or a conversion coating, while parts that see frequent condensation, outdoor exposure or salt need sealed anodize or a conversion coating under powder coat. Copper-bearing 2xxx and 7xxx alloys should never be left bare outside a dry, heated space.
The table uses the corrosivity categories of ISO 9223, which classifies atmospheres from C1 to CX by the first-year corrosion of standard metal specimens and also lets the category be estimated from local conditions such as humidity, temperature, sulfur dioxide and airborne chloride. Use the category the part actually sees, including what happens in transit and in storage, not just its final installed location. Per MIL-A-8625F, Type II anodize is sealed unless the order says otherwise, while Type III hard coat is left unsealed unless sealing is called out, which is why a corrosion-critical hard-coat part needs the word “sealed” on the drawing.
| Service environment (ISO 9223 category) | 6061 or other 5xxx/6xxx | 7075, 2024 or 2011 |
|---|---|---|
| Dry, heated indoor (C1 very low) | Bare acceptable if cosmetic stains do not matter; chem film for grounding faces | Chem film or Type II anodize; avoid bare |
| Unheated indoor, condensation possible (C2 low) | Chem film or Type II anodize, sealed | Type II anodize, sealed; 7075 in T73 or T7351 if stressed |
| Indoor with frequent condensation, or urban outdoor (C3 medium) | Type II anodize, sealed; or powder coat over a conversion coating | Reconsider the alloy; if kept, sealed anodize plus sealant at joints |
| Industrial or coastal (C4 high) | Powder coat over a conversion coating, or sealed anodize with sealed joints | Avoid 2xxx; 7075 only in T73 or T7351 with a full coating system |
| Coastal with high airborne salt, or offshore (C5 very high, CX extreme) | Full coating system over pretreatment; isolate all dissimilar fasteners | Not recommended; redesign in 5xxx or 6xxx where strength allows |
Five practical points sit behind the table. First, finish the part after the last machining operation; MIL-A-8625F already requires parts to be anodized after all heat treatment, machining, welding, forming and perforating unless the order says otherwise, so a tapped hole drilled through anodize is a deviation as well as a bare pit site. Second, small holes and tapped holes can come out of the tank with anything from no film to a full coating, and the same specification cautions designers to call for thread and hole sealing at assembly where corrosion resistance is needed. Third, unsealed anodize is porous, so “anodize” without a type, class and seal requirement is not a corrosion callout; MIL-A-8625F allows scratches to be touched up with a conversion coating only when the damaged area stays within 5 percent of the anodized area, so a heavily scratched lot needs rework, not touch-up. Fourth, anodic coatings develop voids at sharp edges and corners, and powder coat thins there too, so radius edges before finishing and coat powder over a conversion coating, not over bare or merely degreased metal. Fifth, a finish never fixes a design that holds water, so drain holes, sealant in lap joints and fasteners that enter from below belong on the same drawing. The chem film vs anodize guide covers the trade-offs between those two finishes.
How should aluminum parts be stored and packed so they do not corrode before use?
Keep the parts warmer than the dew point of the air around them, keep faces from trapping water against each other, and never let a cold shipment open in a warm, humid room. Most white powder found at receiving formed in the box, not in service.
Condensation forms on metal whenever the metal is colder than the dew point of the surrounding air. The Aluminum Association’s worked example: in a storage area at 13 °C (55 °F) and 60 % relative humidity the dew point is about 4 °C (40 °F), so metal arriving colder than that will collect water (TR3, Table 1). The same guidance tells receivers to let cold metal warm slowly in its unopened package before moving it into heated storage, and to limit moves between areas whose temperatures differ by more than 11 °C (20 °F) to what will be used immediately (TR3, Appendix 1).
For finished machined parts, turn that into a packing instruction:
- Dry the parts completely after the final wash, including blind holes and threads, before they are bagged.
- Separate critical faces with clean interleaving paper or individual pockets so no two bare faces trap water.
- Use sealed bags with a corrosion inhibitor, or a clean water-free preservative where the next process allows it, and say whether the parts will be anodized or painted later, because some preservatives must come off first.
- Handle with gloves after the final clean; fingerprints leave salt.
- At receiving, check for wet or puckered packaging and cold metal, record it on the receiving papers, and let the cartons reach room temperature before opening them.
What should the drawing, RFQ and PO say so aluminum corrosion does not recur?
State the alloy and temper, the finish by specification with type, class and sealing, which features are machined after finishing or masked, the service environment, and the packing requirement. Each of those lines closes one of the causes ranked above.
- Material by alloy and temper. For example, “7075-T7351 PER ASTM B211” rather than “7075,” so a stress-corrosion-resistant temper cannot be swapped for T6.
- Finish by specification. For example, “ANODIZE PER MIL-A-8625, TYPE II, CLASS 1, SEALED” or “CHEMICAL CONVERSION COATING PER MIL-DTL-5541, TYPE II, CLASS 3.” For powder coat, name the pretreatment under it.
- Sequence and masking. “ALL MACHINING COMPLETE BEFORE FINISH” or, where a feature must stay bare, a masking note with the face identified.
- Environment. One line such as “SERVICE: OUTDOOR COASTAL, ISO 9223 C4” lets the supplier flag a mismatch between alloy, finish and use before quoting.
- Joint protection. Sealant, isolation washers or wet installation for any dissimilar fastener.
- Verification. If corrosion performance is a requirement, name the test, its standard and the pass rule; the salt spray guide explains what each finish specification requires and what a pass proves.
- Packing on the PO. Dry before packing, interleave critical faces, inhibitor bags, and gloves after final clean.
When a lot arrives with corrosion anyway, quarantine it, photograph the affected faces next to the packaging, and run the diagnostic table above before deciding between cleaning, refinishing and rejection. Water staining is mostly cosmetic and can sometimes be removed before finishing, while pitting on a sealing or fatigue-critical face usually cannot be accepted without engineering review.
Who does what on a Saola order for a corrosion-sensitive aluminum part?
The machining is done by CNC milling and CNC turning at our lead CNC plant in Vietnam, and the finish is applied by a partner finishing plant we manage. The quote names the production site and country of origin for each step.
Anodizing on aluminum and powder coating are on the surface finishing menu that our partner plants apply; those plants are not named and are not necessarily in Vietnam. Chem film to MIL-DTL-5541 is not on that menu today, so a drawing that requires it, whether as the finish, as a touch-up or as the base under powder, is raised when the RFQ is reviewed rather than quietly substituted. Salt-spray results are supplied on request; name the standard and the exposure you need in the RFQ. Our quality program is working toward ISO 9001, which is in progress and not held today.
To keep the corrosion causes in this guide off your parts, send the alloy and temper, the finish callout with sealing, the masking notes, the environment line and your packing requirements with the drawing. The short manufacturability note that comes with each quote is the place where a risky alloy, finish and environment combination from the table above can be raised. Start through the quote form.
Frequently asked
Is white powder on aluminum harmful, and can it be cleaned off?
The powder itself is mostly aluminum oxide and hydroxide and is not a strength problem on its own. Light staining and residue can usually be cleaned with a non-abrasive pad and clean water, followed by thorough drying. If brushing reveals pits, cleaning does not restore the surface, and the part needs an engineering decision before it is refinished or used.
Does anodizing stop aluminum from corroding?
Sealed anodize greatly improves corrosion resistance, but only where it is intact. Holes machined after anodizing, scratches, contact points and unsealed coatings remain weak spots. Call out the type, class and sealing, and finish after machining.
Why did 7075 parts corrode when 6061 parts in the same box did not?
7075 carries far more copper than 6061, which makes it less resistant to pitting, and in the T6 temper it is also sensitive to stress corrosion and exfoliation. Left bare in the same humid box, it will show corrosion first. Finish 7075 parts, and specify T73 or T7351 when the part is stressed and exposed.
Is chem film enough to protect aluminum outdoors?
On its own, chem film is a light-duty finish best suited to indoor parts, electrical bonding faces and as a base for paint or powder. For outdoor or coastal service, use it under a powder coat or paint, or choose sealed anodize instead.
Should desiccant go into every box of aluminum parts?
Desiccant helps in a well-sealed package, but it has a finite capacity and a leaking or long-haul package can overwhelm it; the Aluminum Association does not recommend desiccants for packaged coil, sheet and plate because they overload quickly on long routes. Dry parts, sealed inhibitor bags and avoiding cold-to-warm transfers matter more than the desiccant itself.
Sources
- TR3, Guidelines for Minimizing Water Staining of Aluminum, fifth edition, The Aluminum Association, May 2009
- ISO 9223:2012, Corrosion of metals and alloys, Corrosivity of atmospheres, Classification, determination and estimation, ISO, 2012
- ASTM G46-21, Standard Guide for Examination and Evaluation of Pitting Corrosion, ASTM International, 2021
- MIL-A-8625F, Anodic Coatings for Aluminum and Aluminum Alloys, US Department of Defense, 10 September 1993
- MIL-DTL-5541F, Chemical Conversion Coatings on Aluminum and Aluminum Alloys, US Department of Defense, 2006
- ASTM B211/B211M-23, Standard Specification for Aluminum and Aluminum-Alloy Rolled or Cold Finished Bar, Rod, and Wire, ASTM International, 2023
- Stress Corrosion of High-Strength Aluminum Alloys, NASA TM X-50924 (MTP-P&VE-M-63-8), T. S. Humphries, NASA Marshall Space Flight Center, June 1963
- Corrosion of Aluminum and Aluminum Alloys, Total Materia, 2001