Visual Signs and What They May Mean
| Appearance | Possible condition | What to check next |
|---|---|---|
| White or gray powdery deposit | Hydrated aluminum corrosion product or dried process residue | Clean a small area and check for surface loss or pitting |
| Small cavities or pinholes | Pitting corrosion | Inspect depth, density and whether chlorides or stagnant moisture are present |
| Thread-like tracks under paint | Filiform corrosion beneath a coating | Check coating damage, edge sealing and humidity exposure |
| Attack near steel, copper or fasteners | Possible galvanic corrosion | Identify both metals, electrical contact and electrolyte path |
| Dark or iridescent stain without metal loss | Water stain, heat tint or surface contamination | Compare cleaned and uncleaned areas; do not classify by color alone |
How to Distinguish Corrosion from Dirt or Oxide Film
- Photograph the area before cleaning and note its location and environment.
- Use a compatible mild cleaning method on a small test area; avoid aggressive abrasion that removes evidence.
- Inspect under good light and magnification for pits, undercut coating or measurable metal loss.
- Check crevices, joints, drainage points and dissimilar-metal contacts.
- For critical parts, use an agreed inspection method and engineering acceptance limit rather than appearance alone.
Common Corrosion Forms in Aluminum
Pitting
Localized pits can form when the protective oxide film is disrupted, particularly in chloride-containing or stagnant environments. A small visible opening may hide a deeper cavity, so critical parts require depth assessment.
Galvanic attack
When aluminum is electrically connected to a more noble metal in the presence of moisture, attack may concentrate on the aluminum near the joint. Isolation, drainage, compatible fasteners and coatings must be considered as a system.
Crevice and coating-related attack
Trapped moisture at laps, gaskets and deposits can create a local environment different from the exposed surface. Under-film tracks or blistering can indicate loss of coating protection.
When Is the Condition Serious?
Escalate inspection when corrosion is on a pressure boundary, thin wall, fatigue-critical feature, joint, electrical contact or dimensionally controlled surface. Cleaning the appearance does not restore lost section thickness. Acceptance must follow the drawing, product standard or responsible engineer’s criteria.
Information to Record for Material Review
- Alloy, temper and product form
- Service fluid, temperature, humidity and contaminants
- Location and dimensions of affected area
- Contact metals, fasteners, sealants and coatings
- Photos before and after controlled cleaning
- Measured pit depth or remaining wall where required
Compare the affected component with the correct alloy, temper and product form before requesting replacement material.
Post time: Jul-16-2025