Tinplate Coil (ETP / MR / L / MC Grade)

Electrolytic tinplate (ETP) is cold-rolled low-carbon steel strip coated on both surfaces with a thin layer of pure tin (Sn) deposited by the electrolytic plating process, producing the world's most widely used metal packaging material for food cans, beverage cans, crown caps, and general metal packaging. The term 'electrolytic' distinguishes modern tinplate from historical 'hot-dipped' tinplate where the steel was passed through a bath of molten tin — electrolytic plating provides precise coating weight control, uniform thin coatings achievable in the range 1–11 g/m², and high production speed that hot-dip tinning cannot match. Tin coating weight is specified in grams per square metre (g/m²) per side, expressed as a pair of values representing the coating on each surface. The two main coating weight systems are: EN 10202 / European designation: expressed as 'E' + coating weight in g/m², e.g., E2.8/E2.8 means 2.8 g/m² on each side (equal coating), or E5.6/E2.8 means 5.6 g/m² on the inside (food-contact side) and 2.8 g/m² on the outside (differential coating). ASTM / US designation: expressed as '#' + numeric code where each number represents 1.12 g/m² per lb/BB (pound per base box, the traditional US tinplate unit), e.g., #25/#25 = 2.8 g/m² per side. Standard commercial coating weights include: 1.0/1.0 g/m² — very light coating for dry product packaging or exterior-only use; 2.8/2.8 g/m² — the dominant global standard for food can bodies and ends providing adequate protection for most canned food products under 3–5 year shelf life; 5.6/5.6 g/m² — heavy coating for corrosive food products (high-acid fruit, sulfurous vegetables, fish in brine); 8.4/8.4 g/m² — maximum standard coating for the most corrosive applications and longest shelf life (10+ years military rations, long-term emergency food storage). Differential coating (5.6/2.8, 8.4/2.8) is commercially important — heavier coating on the can interior (food-contact side) provides enhanced corrosion protection from the product, while standard coating on the exterior reduces tin consumption and cost since the exterior is protected by the printed lacquer system.

Tinplate steel grade designations (MR, L, D, MC in international standards; corresponding to grades in JIS G3303 and EN 10202) classify the steel substrate by its residual element content — primarily copper, nickel, chromium, and molybdenum — which affects the corrosion behaviour of the steel in the presence of food products and the solderability and weldability performance of the tinplate in can making operations. MR Grade (Most Restrictive, also called 'single reduced base' in some market designations): The most widely used commercial tinplate steel grade, with controlled residual element limits — copper ≤0.10%, nickel ≤0.06%, chromium ≤0.06%, molybdenum ≤0.01% per typical market specifications. MR grade steel is suitable for the full range of food packaging applications including sulfurous foods (fish, meat, pet food) where the steel substrate must not contribute sulfur-reactive elements that could form black FeS discoloration at pitting defects in the tin coating. MR grade tinplate is the standard specification for three-piece food can bodies, food can ends, DRD two-piece cans, and crown caps for the full range of food product applications. L Grade (Low residual): The most stringent residual element specification, with copper ≤0.05%, nickel ≤0.04%, chromium ≤0.04%, and other residuals at even lower limits than MR grade. L grade steel is specified for the most sensitive food applications including infant formula packaging, baby food, and premium canned foods where the absolute minimum level of metallic contamination potential is required and where the steel's corrosion behaviour must be most predictable and controlled over long shelf life. L grade commands approximately 5–8% price premium over MR grade. D Grade (Drawing quality for difficult can designs): Steel with controlled inclusion content and improved deep drawability characteristics for two-piece DRD can applications requiring the most severe forming deformation, aerosol can body production requiring deep draw and subsequent re-forming operations, and crown cap stock applications where high press speed stamping performance is critical. The D designation addresses the steel's formability and consistent mechanical properties for difficult stamping rather than residual element content like MR and L grades. MC Grade (Medium carbon): A less common grade specification for applications requiring specific mechanical property characteristics achievable by slightly higher carbon content than standard MR grade — used in some specialty aerosol and general line can specifications where higher hardness is required from SR tempers.

Tinplate food can interiors are coated with organic lacquer systems that provide the primary barrier between the steel/tin substrate and the packaged food product, preventing metallic contamination of the food, inhibiting corrosion reactions between the tin and food acids, controlling the dissolution rate of tin into the food product to within food safety limits, and in some applications providing the sealing function at the ERW seam area where the tin coating is disrupted by the welding process. The principal interior lacquer systems for tinplate food cans are: Epoxy-Phenolic Lacquers: The most widely used interior can lacquer globally, providing excellent adhesion to tinplate, outstanding resistance to the steam retort sterilisation process (121°C, 15–30 minutes at 2–3 bar), good chemical resistance to both acidic and neutral food products, and very low migration of lacquer components into the packaged food. Epoxy-phenolic is the standard interior coating for vegetable cans, meat cans, fish cans, pet food cans, soup cans, and all canned food products processed by retort sterilisation. Applied to tinplate coil or sheet by roll coater at typically 2–6 g/m² dry film weight, then oven-cured at 180–220°C. Vinyl Organosol Lacquers: A vinyl chloride / vinyl acetate copolymer dispersion providing excellent flexibility and extensibility for DRD drawn can applications, outstanding resistance to sulfur compounds (preventing tin sulfide blackening in fish, egg, and meat products), and very good retort resistance. Vinyl lacquers are the preferred choice for fish cans (sardines, tuna, salmon, mackerel), pet food cans, and other sulfurous product applications where standard epoxy-phenolic lacquers would develop black discoloration from sulfur reaction with the tin coating. PET Film Laminate: An alternative to liquid lacquer where a biaxially oriented polyethylene terephthalate (PET) film (12–25μm thickness) is heat-laminated directly to the tinplate surface before can making, providing a continuous pin-hole-free coverage impossible to achieve with liquid lacquer at practical application weights. PET laminated tinplate (FLTS — Film Laminated Tinplate) is used for high-production two-piece DRD fish and pet food cans where the continuous film eliminates the in-can corrosion failure mode of holidays (pinholes) in roll-coated liquid lacquer. Acrylic lacquers: Used as exterior can decoration base coats and some interior applications for specific product compatibility. Polyester lacquers: Used for some application areas requiring specific chemical resistance or food regulatory compliance characteristics. For ASME food safety and EU food contact material compliance, all lacquer systems used on food-contact tinplate must comply with EU Regulation (EU) No. 10/2011 on plastic materials in contact with food (for polymer-based coatings), EU Regulation (EU) No. 1935/2004 (Framework Regulation on food contact materials), and FDA 21 CFR regulations for the US market.

Passivation of tinplate is a post-plating surface treatment that forms a thin protective layer over the bright reflowed tin surface to prevent tin oxidation (dulling/tarnishing) during storage and shipment before can making, and to improve the adhesion of lacquer and printing inks applied to the tinplate during can production. Without passivation, freshly reflowed tinplate surface would gradually develop a dull grey oxide film (stannic oxide, SnO₂) on storage — particularly in humid conditions — that would reduce the bright metallic appearance, impair lacquer adhesion, and reduce the solderability needed for some can seam applications. The standard commercial passivation systems are: Type 300 (Standard Cathodic Chromate Passivation): The dominant passivation system historically applied to almost all commercial tinplate worldwide. The reflowed tinplate strip passes through a dilute sodium dichromate (Na₂Cr₂O₇) electrolyte tank where cathodic current deposits a mixed layer of metallic chromium (Cr⁰) and chromium oxide / hydroxide on the tin surface, with total chromium deposit typically 5–15 mg/m². Type 300 passivation produces a thin, nearly invisible coating that provides excellent tin surface protection during storage, improves lacquer adhesion significantly versus unpassivated tin, and has excellent compatibility with all standard tinplate lacquer systems and printing ink formulations. Type 311 (Modified Cathodic Chromate Passivation): A variant of Type 300 with modified electrolyte chemistry or deposition conditions producing a slightly different passivation layer composition — used by some producers to achieve specific lacquer adhesion performance characteristics or compatibility with particular customer lacquer systems. Both Type 300 and Type 311 use hexavalent chromium (Cr⁶⁺) in the dichromate electrolyte, which creates a REACH regulation challenge — while the final passivation layer on tinplate contains primarily trivalent Cr⁰ and Cr³⁺, the process electrolyte itself contains hexavalent chromate. Alternative Passivation (Type D and Cr-Free): In response to REACH regulatory pressure on hexavalent chromium process chemicals, several tinplate producers have developed or are developing chrome-free passivation systems using alternative chemistry (organic passivation compounds, zirconium-based passivation, or other non-chromium technology) that achieve equivalent tin surface protection and lacquer adhesion without chromium compounds in the process electrolyte. These systems are increasingly specified by environmentally focused can makers and consumer brand owners with sustainability commitments, though they currently represent a small percentage of global tinplate production. Chemical Passivation (Type 300C — alternative designation): In some specifications, chemical passivation by immersion in dichromate solution without electrolytic current is an alternative to cathodic passivation, producing a thinner and less consistently uniform passivation layer than cathodic treatment. Oiling over passivation: Standard practice to apply food-grade lubricant oil (DOS — dioctyl sebacate, ATBC — acetyl tributyl citrate, or ATO — acetyl trioctyl citrate) at 1–3 mg/m² per side over the passivation layer to reduce friction and tooling wear during high-speed can making and to provide a thin barrier against further atmospheric contact with the passivated tin surface during storage and transit.

Single-reduced (SR) and double-reduced (DR) tinplate differ in the number of cold rolling stages applied to the steel substrate before tinplating, producing significantly different mechanical property levels that determine application suitability. Single-Reduced (SR) tinplate is produced by the conventional sequence: hot-rolled coil → pickle → cold roll to substrate thickness → anneal (continuous strip anneal or box anneal) → temper roll (skin pass 1–2%) → electrolytic tinplating. The anneal step recrystallises the cold-rolled microstructure into equiaxed grains providing balanced strength and ductility. SR tempers T1 through T5 range from yield strength ~220 MPa (T1, softest, for deepest draw) to ~360 MPa (T5, for ends requiring panel stiffness), with elongation from ≥28% (T1/T2 for can body forming) to ≥15% (T5). SR tinplate is the standard specification for: three-piece food can bodies (T2 and T3, requiring roll forming, ERW seam welding, flanging, beading, and double seaming operations); DRD two-piece drawn can bodies (T1 and T2, requiring severe drawing reduction to form the cup shape); aerosol can bodies (T3, requiring re-forming operations after initial drawing); and crown caps (T4, requiring the hardness to resist deformation from bottle internal pressure at the minimum thickness for crown geometry). Double-Reduced (DR) tinplate is produced by adding a second cold rolling stage without intermediate anneal after the base anneal: → base anneal → second cold roll at 20–40% reduction → temper roll → electrolytic tinplating. The second cold rolling pass work-hardens the steel substantially beyond the SR hardness range, increasing yield strength (DR8: ≥415 MPa, DR9: ≥440 MPa, DR10: ≥460 MPa) while reducing elongation to ≤8% — too low for most forming operations but adequate for the limited deformation of can end stamping. The higher yield strength of DR steel allows production of thinner end stock (0.13–0.21mm for DR9 versus 0.14–0.25mm for T4 SR) with equivalent panel resistance to buckling under internal can pressure — since the end panel stiffness depends on both yield strength and plate thickness (panel buckling strength approximately proportional to σ_y × t²), the higher yield strength of DR9 enables proportional thickness reduction while maintaining the same pressure resistance. This enables can end diameter, and particularly easy-open end (EOE) dimensions, to be produced from thinner and lighter gauge steel for the same pressure rating — reducing steel consumption per end and therefore per filled can. DR tinplate is the standard specification for: food can ends (both conventional non-easy-open ends and EOE ring-pull ends — DR9 at 0.18–0.21mm is the dominant end stock temper/gauge combination globally); beverage can ends for steel DWI cans (DR9 and DR10 for the thinnest possible end stock consistent with carbonation pressure panel buckling resistance); and crown caps (DR8 in some specifications as a higher-strength alternative to T4 SR for minimum gauge crowns).