I. Rust Symptoms
1. Iron/Steel Cans
Initially, dark gray spots appear, which develop into a reddish-brown or brownish-yellow loose material (primarily composed of iron oxide). In severe cases, pits or scars may form. Scraping away the rust reveals a dark gray base.
If treated with bluing or phosphating, the rust may appear yellowish-brown, contrasting with the original dark-gray surface.
2. Other Metals
Copper cans: bluish-green rust (basic copper carbonate) or orange-red spots (cuprous oxide).
Aluminum cans: a white or gray powdery oxide layer (aluminum oxide), which is usually dense and harmless.
Stainless steel cans: brown rust (caused by the destruction of the chromium oxide layer), often seen on scratched or stained areas.
II. Inspection Methods
1. Internal Rust
Visual Inspection: After opening the can, inspect the interior for reddish-brown spots, loose material, or pitting. If paint residue obscures rust, clean the surface first. Touch Test: Rust is typically rough and easy to peel, while dirt or water stains feel smooth to the touch.
2. Determining Exterior Rust
Welds and Seams: Focus on areas prone to moisture accumulation, such as the lid threads and the welds on the can body. Rust often appears as brown, banded marks.
Coating Condition: Bubbling, cracking, or flaking areas of the exterior paint may conceal rust. Scrape the paint to inspect the underlying metal.
III. Rust Severity Grading
Grade A (Mild): Surface oxidation only, no visible rust.
Grade B (Moderate): Localized rust with partial flaking of the mill scale.
Grade C (Severe): Extensive rust with pitting.
Grade D (Severe): Complete flaking of the mill scale, resulting in structural damage to the can.
IV. Precautions
1. Plastic cans will not rust, but they should be inspected for deformation or deterioration.
2. Rusted cans may contaminate the paint; thorough cleaning or replacement is recommended. 3. In coastal or high humidity environments, the inspection cycle needs to be shortened (once a month is recommended).
