Not every post-dispatch issue is a process failure
Coating failures can originate in processing, design, packaging, transport, storage or customer handling. A systematic diagnosis is needed before assigning cause. The location, pattern and timing of corrosion or damage are important clues.
Uniform coating breakdown on significant surfaces may indicate process or specification issues. Localized marks at contact zones, cardboard interfaces, wet pockets or fingerprints often point toward handling and storage.
Moisture patterns
White corrosion in stacked zones, washer interfaces, thread pockets or carton-contact areas often indicates trapped moisture. This is common during monsoon, long-distance transit, warm packing or warehouse condensation.
Packaging photographs, receipt condition and time from dispatch to inspection help determine whether moisture exposure occurred after processing.
Abrasion and impact damage
Scratches, rubbed high spots and exposed edges often come from bulk packing, vibration, heavy pack weights or part-on-part movement. Xylan/PTFE and zinc flake films can be damaged by sharp threads or hard stacking.
Use dividers, trays, lower pack weights and better cushioning for sensitive parts.
Chemical contamination
Alkaline cardboard glue residues, acidic vapours, shop chemicals, coolant, cutting oil and dirty gloves can attack or stain coatings. Zinc-based coatings are especially sensitive to non-neutral contamination.
Customers should avoid repacking finished parts in used chemical cartons or storing them near acids/alkalis.
Diagnosis clues
| Observed issue | Likely investigation direction |
|---|---|
| Corrosion only where part touched cardboard | Packaging/contact contamination and moisture. |
| Marks at all high spots | Abrasion during packing or transport. |
| Corrosion in blind holes/recesses | Moisture retention, drainage or geometry issue. |
| Finger-shaped stains | Bare-hand handling or salt contamination. |
| Uniform failure across significant surfaces | Process, specification or severe exposure investigation needed. |
Practical conclusion
Good failure diagnosis looks at pattern, location, timing and handling history—not only the coating name.
How buyers should use this guide in an RFQ
For a technically complete coating RFQ, the customer should provide the drawing, material, quantity, current surface condition, required coating system, thickness or coating-mass expectation, salt spray target, masking requirement and packaging expectation. For zinc flake, Xylan/PTFE and phosphating work, route selection cannot be separated from geometry, surface preparation and post-coating handling.
If the part has threads, internal drives, blind holes, seal faces, bores or close-tolerance assembly zones, those areas should be marked before sampling. Coating build-up, retained coating, masking witness marks and post-curing handling must be accepted or corrected during sample approval rather than after bulk production.
Common avoidable rejection causes
- Approving corrosion performance without checking actual assembly, torque, thread fitment or bore clearance.
- Using ordinary cardboard or recycled paper directly against zinc-based coatings during humid storage.
- Bulk-packing Xylan/PTFE or zinc flake parts so that sharp edges and threads abrade the coating during transport.
- Leaving freshly blasted steel exposed before coating, causing flash rust or surface contamination.
- Treating salt spray hours as a universal field-life guarantee without considering storage, handling and exposure conditions.
Documentation and approval discipline
For controlled coating work, approval should include coating route, surface preparation method, number of coats, curing condition, measurement method, visual standard and packaging method. If the customer later changes part geometry, incoming condition, dispatch packing, storage duration or fitment requirement, the approved coating route should be revalidated before production continuation.