Sulfide Stain Resistant Electrolytic Tinplate Coil MR Grade For Fish Meat Paste Cans

Protein rich foods are the harshest environment a can interior will ever face. During retorting, sulphur bearing amino acids break down and, in the presence of iron, form iron sulphide deposits that stain the can wall dark brown or black. The problem is cosmetic in some packs and a genuine quality complaint in others, and it is the single most common reason for rejection of fish and meat cans in export markets. Sulfide stain resistant electrolytic tinplate addresses the issue at the metallurgical level rather than relying on thicker enamel alone.

The substrate is MR grade steel selected for low residual copper and sulphur content, because both elements accelerate sulphide formation. A 5.6 g/m2 tin coating per side provides a substantially larger sacrificial reserve than standard 2.8 g/m2 material, so the coating continues to protect exposed areas such as score lines, beading and seam transitions long after a lighter coating would have been consumed. Surface roughness and passivation are controlled within a narrow band to give internal lacquers a mechanical key that survives the retort cycle without blistering or undercutting.

• Sulfide Stain Control: Low residual copper and sulphur in the MR substrate, combined with a heavier 5.6/5.6 g/m2 tin coating, suppresses the iron sulphide discolouration that turns protein-rich packs black after retorting, so can interiors and exposed score lines stay bright through the full shelf life.
• Enamel Adhesion After Retort: Surface topography and passivation are tuned for lacquer bonding. Internal enamel resists blistering, undercutting and detachment during the 121 degrees Celsius retort cycle, protecting the tin layer where the coating is thinnest at the can bead and expander rings.
• Heavy Tin Coating For Aggressive Packs: The 5.6/5.6 g/m2 coating provides roughly double the sacrificial tin reserve of standard 2.8 g/m2 material, extending corrosion protection for acidic fish sauces, tomato based meat products and salted aquatic packs.
• Consistent Formability: T4 temper delivers the balance of stiffness and elongation required for three piece body rolling, beading and end seaming, with uniform mechanical properties across the coil so that double seam tightness does not vary from the head to the tail.
• Verified Stain Testing: Every production lot is checked using an internal sulfide stain test at 121 degrees Celsius for ninety minutes on lacquered coupons. Results are recorded against the heat number and can be supplied with the mill certificate for customers who audit incoming material.
• Optimised Coil Build: Coils are wound with controlled tension and edge alignment to prevent telescoping and edge damage in transit, and are available in 660mm to 929mm widths matched to common can body blank sizes, cutting trim loss on the customer line.

The material is specified for three piece and two piece cans holding fish in brine, oil or tomato sauce, sardines and mackerel, tuna chunks, meat paste and luncheon meat, corned beef, pet food and aquatic pastes. It is equally suitable for other sulphur bearing products such as certain vegetable purees and egg based packs where staining would be visible on opening.

Can makers typically combine the coil with an internal lacquer system and a side stripe, and benefit from the extra tin reserve where the coating is unavoidably thin at the bead and the expander rings. Because coil widths from 660mm to 929mm are stocked, blank optimization for standard body sizes is straightforward. Trial coils with accompanying sulfide stain test data can be supplied for can maker qualification and customer approval before container orders are placed.

Each coil is wrapped in anti-corrosion VCI film, interleaved with desiccant bags, then sheathed in kraft paper and finally enclosed by galvanised steel sheet with reinforced corner protectors. Coils are stacked eye-to-sky or eye-to-wall according to customer unloading equipment, strapped with PET bands at four points, and loaded into 20ft or 40ft containers on fumigated wooden skids. Sheet bundles are protected by edge boards and plastic strapping so that the surface finish reaches the destination port free of scratches, white rust and transit dents.

The mill operates under an ISO 9001 certified quality system. Every heat number is traceable and each shipment is accompanied by a mill test certificate covering chemical composition, tensile and yield strength, tin coating weight measured by coulometric stripping, and surface roughness. Third-party inspection by SGS, BV or Intertek can be arranged on request before container sealing, and a retained sample from each lot is archived for twelve months for future claim investigation. Any dimensional or coating non-conformity is settled by replacement or price adjustment within the contracted tolerance framework.