Why Biscuit Tins Rust: 5 Manufacturing-Level Insights Most Buyers Never See

Rust on biscuit tins is often blamed on poor storage or humid environments.

That explanation is convenient — but incomplete.

From a tin box manufacturer’s perspective, rust is rarely an accident.

In most cases, it is the result of material selection, coating decisions, and overlooked production details.

This article explains why biscuit tins rust, not from a consumer angle, but from the standpoint of manufacturing engineers and quality inspectors who work with food tin packaging every day.

If you are sourcing from biscuit tin manufacturers or evaluating a long-term supplier, these details matter more than surface appearance.

1. Rust Starts with Tinplate Choice — Not All “Tin” Is the Same

Most biscuit tins are made from Electrolytic Tinplate (ETP).

That part is standard. The difference lies in what buyers rarely ask about:

Tin Coating Weight

Common options in the industry include:

• 2.0 / 2.0 g/m²
• 2.8 / 2.8 g/m²
• 5.6 / 5.6 g/m²

For dry snacks, lower coating weights can work — but biscuits are not neutral products.

They contain:

• sugar residues
• fats
• micro moisture after baking

From experience, biscuit tins using coating weights below 2.8 / 2.8 g/m² are significantly more prone to early oxidation during sea transport or warehouse storage.

Buyer reminder:
If a supplier avoids specifying coating weight, it is usually for cost reasons — not technical ones.

2. Internal Lacquer Is Not Decorative — It Is Functional Protection

Many food tin manufacturers advertise “food-grade coating”.

What they don’t explain is where and how consistently it is applied.

Gold Lacquer vs. Clear Lacquer

For biscuit tins, food-grade gold lacquer is commonly used because:

• it forms a stronger barrier against sugar-induced corrosion
• it tolerates long storage cycles better than clear lacquer

However, lacquer performance depends on coverage continuity, not color.

A thin or uneven lacquer layer creates microscopic exposure points.

Those points become corrosion starters once humidity enters the system.

Small insight from production:

Rust rarely starts on flat panels.

It starts at edges, seams, and stressed areas.

3. The Side Seam Problem Most Biscuit Tin Suppliers Don’t Mention

This is one of the most common — and least discussed — causes of rust.

Unprotected Side Seams

To reduce cost, some factories:

• coat the main body
• coat the lid and bottom
• skip secondary protection on the internal side seam

That seam is where:

• tinplate is mechanically stressed
• coating continuity is easiest to break

When biscuit crumbs absorb moisture, corrosion often begins from the inside seam, not from the visible surface.

This is why some tins look perfect externally, yet rust appears internally within months.

Quality check tip:
Ask how the side seam is treated after forming — not just what lacquer is used.

4. Biscuit Tins Fail at the Edges, Not the Panels

Another overlooked factor is edge exposure.

During forming, the following areas are most vulnerable:

• lid curl edges
• bottom curl areas
• cut edges around openings

If these edges are not properly sealed or recoated, the tin coating is locally weakened.

For cookie tin box manufacturers, this is a known risk area — but it requires extra process steps that some suppliers skip.

Important distinction:
A visually smooth edge is not the same as a chemically protected edge.

5. Salt Spray Testing: What It Actually Proves (and What It Doesn’t)

Many buyers hear about Salt Spray Test results but don’t know how to interpret them.

Why Biscuit Tins Are Tested

Salt spray testing simulates:

• high-humidity exposure
• aggressive corrosion conditions
• long shipping and storage cycles

For biscuit tins, a 24-hour test is a baseline.

Higher-end food brands often require 48 hours, especially for export markets.

What the Test Really Reveals

Salt spray testing exposes:

• lseam protection failures
• edge coating weaknesses
• lacquer discontinuity

It does not test appearance.

It tests manufacturing discipline.

Buyer reminder:
A supplier who can show test results usually has stable process control.

A supplier who avoids testing often relies on appearance alone.

6. Rust Is a System Failure — Not a Single Defect

When biscuit tins rust, it is rarely due to one factor alone.

It is usually a combination of:

• insufficient tin coating weight
• incomplete lacquer coverage
• untreated seams or edges
• lack of corrosion testing

From a tin packaging manufacturer’s viewpoint, rust prevention is a system decision, not a single upgrade.

7. What Biscuit Buyers Should Ask Their Tin Manufacturer

If you are sourcing from biscuit tin manufacturers or evaluating a new supplier, these questions are more effective than price comparisons:

• What tin coating weight is used for biscuit tins?
• How is the internal side seam protected?
• Which food-grade lacquer is applied, and where?
• Do you perform salt spray testing? At what duration?
• Are production samples inspected at seam and edge areas?

These answers reveal far more than a polished product photo.

Final Thought from the Manufacturing Side

Rust is not a mystery.
It is a consequence of decisions made during material selection and production.

A biscuit tin that resists oxidation is not necessarily expensive —

but it is intentionally engineered.

That difference is what separates a reliable tin box manufacturer from a supplier focused only on short-term cost.