Tinplate Sheet — T1 / T2 / T3 / T4 / T5 (Electrolytic Tinplate / ETP)

Tinplate temper grades define the hardness and strength of the steel base, determined by the degree of cold work hardening and annealing applied during manufacturing. Selecting the correct temper is critical for can-making performance — too soft a temper and the can panel lacks rigidity; too hard a temper and the material cracks during drawing operations.

Single-Reduced (SR) Grades T1 through T5:
These grades are produced by a single cold rolling pass to final thickness, followed by annealing and skin pass rolling. The temper number reflects increasing hardness from T1 (softest) to T5 (hardest):

T1 (HR30T = 46±3): The softest tinplate grade with maximum formability and elongation. Used for deep-drawn components requiring severe deformation without cracking — complex drawn tin box shapes, conical aerosol dome tops, drawn and ironed (DWI) can bodies, bottle cap liners, and pharmaceutical tin components with complex contoured shapes. Lowest strength limits T1 to applications where panel rigidity is provided by the can geometry (convex shapes) rather than material strength.

T2 (HR30T = 50±3): Moderate formability with slightly higher strength than T1. Primary application is decorative tin box manufacturing (biscuit tins, gift tins, confectionery tins) where the hinged lid forming, edge seaming, and embossing operations require controlled formability, and printed decorative external surfaces require a smooth, defect-free tin surface that T2's softer base provides more reliably than harder grades.

T2.5 (HR30T = 52±3): Intermediate grade between T2 and T3 for applications requiring more strength than T2 but better formability than T3. Used for food can bodies of smaller diameter and paint can bodies with moderate draw depth.

T3 (HR30T = 56±3): The most widely produced and consumed tinplate grade globally — the standard specification for the majority of three-piece food can bodies (tomatoes, vegetables, fish, pet food), general packaging, and aerosol can bodies where balanced formability and panel rigidity are required. T3 provides the optimal combination of ease of forming on high-speed can-making lines and sufficient rigidity for stacked can handling and retail display.

T4 (HR30T = 60±3): Higher strength for can components requiring greater rigidity — can end stock for smaller-diameter easy-open ends, aerosol can bodies requiring higher hoop strength, crown cap stock, and general packaging where higher pressure or stacking loads are involved.

T5 (HR30T = 64±3): Hardest single-reduced grade for maximum rigidity in can ends and crown caps. Used for full-open easy-open ends of larger-diameter food cans (where the larger diameter requires higher end panel strength to resist vacuum bulging during sterilization), crown cap stock (requires hardness to resist deformation during cap application at high speed), and metal screw caps.

Double-Reduced (DR) Grades DR8 through DR10:
DR tinplate receives an additional cold rolling pass (20-40% reduction) after the primary annealing step, achieving significantly higher hardness and yield strength (450-620 MPa) than any single-reduced grade while maintaining thinner gauge. DR grades are used primarily for can end stock and ring-pull tab stock where the very high strength allows thinner gauge to be used without compromising end panel rigidity — each gram of tinplate saved per end × billions of cans produced per year represents enormous material cost savings:

DR8 (HR30T = 73±3, yield ~450-520 MPa): Standard double-reduced grade for beverage can end stock, easy-open can ends for food cans, and ring-pull tab stock. Enables 10-15% gauge reduction versus equivalent T5 SR end stock.

DR9 (HR30T = 76±3, yield ~500-570 MPa): Higher strength DR grade for ultra-thin easy-open ends and lightweight can design programs where maximum material efficiency is the priority.

DR10 (HR30T = 80±3, yield ~540-620 MPa): Maximum strength DR grade for the thinnest possible can end and tab stock — used by leading can-makers pursuing lightweighting programs to minimize tinplate consumption per can unit.

Tin coating weight designates the mass of pure tin deposited per unit area on each surface of the tinplate, expressed in grams per square meter (g/m²) per surface. The coating weight designation (E2.8, E5.6, E8.4, etc.) indicates the minimum average tin coating weight in g/m² that must be present on each surface of the tinplate as verified by X-ray fluorescence (XRF) measurement per EN 10202. Understanding tin coating weight selection is essential for can-makers and packaging engineers to ensure adequate corrosion protection for the packed product without over-specifying tin content (higher tin weight = higher cost).

Coating Weight Designation System (EN 10202):
EN 10202 uses E-class designation: E1.0, E1.5, E2.0, E2.8, E4.0, E5.6, E8.4, E11.2 — where the number indicates the minimum average tin coating weight in g/m² per surface. Equal coating (both surfaces same): designated as E2.8/2.8 (same coating weight on both sides). Differential coating (inner surface higher tin for corrosion protection, outer surface lower tin to minimize cost): designated as E5.6/2.8 (inner surface 5.6 g/m² minimum, outer surface 2.8 g/m² minimum). The outer/external surface of a can always receives the lower tin coating weight in differential coating specifications.

Corrosion Protection and Coating Weight Selection:
The tin layer provides corrosion protection through two mechanisms: physical barrier (tin metal separating food from steel) and electrochemical protection (in most foods, tin is anodic to steel — tin corrodes sacrificially to protect the steel substrate, preventing iron dissolution into food). Higher tin coating weight provides: longer time before tin coating is consumed (longer shelf life), better protection against pinholes that expose steel base, higher tolerance for aggressive food products.

Food Product Corrosivity and Recommended Coating Weight:

Low corrosivity (2-3 year shelf life): Water-based products, dairy products (condensed milk, evaporated milk — relatively neutral pH), dried fruits in syrup, some vegetables. Recommended: E2.8/2.8 equal coating, or E2.8/2.8 with internal lacquer. This is the minimum standard coating for most processed food applications.

Moderate corrosivity: Meat products (stew, corned beef, luncheon meat — proteins and fats provide some protection), soups and broths, legumes (peas, beans — moderate sulfur content), some fruits in syrup. Recommended: E2.8/2.8 with appropriate internal lacquer system, or E5.6/2.8 differential coating (higher tin on food contact inner surface).

High corrosivity: Tomatoes and tomato products (pH 3.8-4.5, high organic acid content — the most aggressive standard food product for tinplate), citrus fruits and juices (high citric acid, low pH), tropical fruits (pineapple — bromelain enzyme activity), carbonated beverages in tinplate. Recommended: E5.6/2.8 differential coating with appropriate internal lacquer (epoxy phenolic or equivalent), or E8.4/2.8 for extreme cases.

Very high corrosivity (sulfur-staining risk): Fish and seafood (tuna, mackerel, sardines, salmon — hydrogen sulfide production during heat processing causes black tin sulfide staining on unprotected tin surface), pet food (high protein, high sulfur). Recommended: E8.4/2.8 differential coating with C-enamel lacquer (containing zinc oxide pigment that reacts with hydrogen sulfide to form white zinc sulfide instead of black tin sulfide), providing both barrier protection and stain prevention.

Non-food applications (paint, chemicals, aerosol): Tin coating weight selection based on the corrosivity of the contained product toward steel — solvent-borne paints typically need lacquer-lined tinplate (E2.8 coating), water-borne latex paints can use unlined tinplate (E2.8-E5.6), aggressive solvents and acids require E5.6-E8.4 with appropriate chemical-resistant lacquer. Tanglu Group provides coating weight selection guidance based on product type and target shelf life — contact our technical team with your product formulation information for a recommendation.

Differential tin coating is a tinplate configuration where the tin coating weight on the two surfaces of the tinplate differs — the food-contact inner surface receives a higher tin coating weight for corrosion protection against the food product, while the external surface receives a lower tin coating weight since it will be protected by external lacquer or lithographic printing rather than relying on tin corrosion resistance alone. Differential coating is one of the most important tinplate specification parameters for can-makers, as it allows optimization of tin usage and cost per can unit without compromising food safety or product shelf life.

How Differential Coating is Produced:
During electrolytic tinplating, the strip passes through plating cells where the current density can be independently controlled for the upper and lower surfaces (using separate upper and lower anode arrays with independent rectifier circuits). By applying higher current to the food-contact surface during plating, a higher tin deposition rate is achieved on that surface while the external surface receives less current and lower tin deposition — producing differential coating in a single continuous plating pass. The differential coating configuration is specified at the time of mill order and cannot be changed after production.

Designation of Differential Coating (EN 10202):
Differential coating is designated as E(inner)/E(outer) — for example:
E5.6/2.8: Inner surface (food contact) 5.6 g/m² minimum tin, outer surface 2.8 g/m² minimum tin
E8.4/2.8: Inner surface 8.4 g/m² minimum, outer surface 2.8 g/m² minimum
E8.4/5.6: Higher tin on both surfaces for extreme corrosivity products

Identification of Coated Surfaces at Can-Maker:
Differential-coated tinplate must be correctly oriented when fed into the can-making line so the higher-tin surface becomes the food-contact inner surface. Identification methods include: edge marking with ink stripe on the higher-tin edge (standard EN 10202 marking practice), different surface appearance (slightly different brightness or matte character visible under raking light inspection), XRF spot measurement by quality control at the can-maker receiving inspection, and coil/sheet clearly marked with 'inner surface' identification on the mill certificate and packaging labels.

Economic Benefit of Differential Coating:
For a standard food can using E5.6/2.8 differential coating versus E5.6/5.6 equal coating, the tin saving is 2.8 g/m² × 2 surfaces (inner and outer area of can body) for the external surface reduction. At tin metal price of approximately USD 25,000-35,000 per metric ton and tinplate consumption of 500-600 kg per million cans, the tin cost saving from differential coating versus equal high-tin coating represents USD 3,500-5,000 per million cans — a significant cost reduction for high-volume can production programs.

Application Recommendations:
Use equal low coating (E2.8/2.8) for: general food products with low corrosivity, non-food applications where lacquer provides primary protection, decorative tins with lacquer on both surfaces.
Use differential coating (E5.6/2.8) for: standard corrosivity food products (vegetables, meat, soups) where higher inner tin improves corrosion barrier without over-specifying outer surface.
Use high differential coating (E8.4/2.8 or E8.4/5.6) for: highly corrosive food products (tomatoes, citrus, tropical fruit) where maximum inner surface tin is needed for target shelf life, and outer surface may need E5.6 if external corrosion in humid storage is a concern.

Internal lacquer (enamel) systems are applied to the interior of tinplate food cans by can-makers before filling to provide an additional barrier between the food product and the tinplate surface, enhancing corrosion protection, preventing flavor and color interaction between metal and food, and enabling use of lighter-weight tin coatings while maintaining food safety and shelf life performance. Understanding lacquer systems helps in specifying the correct tinplate for lacquered can applications.

Why Internal Lacquers Are Used:
Although tin coating alone provides corrosion protection for low-corrosivity foods, many food products — particularly those with high acidity, high sulfur content, or strong pigments — require an additional organic coating barrier to: prevent tin dissolution into food product exceeding regulatory limits (EU: 200 mg/kg maximum tin in canned food), prevent iron dissolution that causes rust discoloration, prevent sulfur compounds from reacting with tin to form black tin sulfide staining (fish, pet food), prevent flavors and colors from the food leaching into or reacting with the tin surface (fruit juices, tomato products), and extend product shelf life beyond what tin coating alone can reliably protect.

Major Internal Lacquer Systems Used in Food Can Manufacturing:

1) Epoxy Phenolic Lacquer (EP Lacquer):
The most widely used internal lacquer for food cans globally. Two-component thermosetting system (epoxy resin crosslinked with phenolic hardener) baked at 190-210°C for 10-12 minutes on a roller coater or spray coater. Excellent adhesion to tinplate, outstanding chemical resistance to most food acids, oils, fats, and aqueous food products. Film thickness 3-7 g/m² dry film weight, provides excellent barrier and resistance to retort sterilization temperatures (121°C for 90 minutes without film degradation). Standard internal lacquer for vegetables, meat, soups, fruit products. EU food contact compliance per Regulation 10/2011 with migration testing required — BPA (bisphenol A) free versions increasingly specified by major food brands due to consumer concerns about epoxy BPA migration.

2) C-Enamel (Zinc Oxide Lacquer):
Epoxy phenolic lacquer base with zinc oxide (ZnO) pigment addition (typically 5-15% ZnO in dried film). The zinc oxide reacts preferentially with hydrogen sulfide produced during heat processing of fish and seafood, forming white zinc sulfide (ZnS) instead of allowing black tin sulfide (SnS) formation. Specifically specified for: tuna cans, sardine cans, mackerel cans, salmon cans, pet food cans (particularly tuna and salmon-based pet food). C-enamel prevents the unsightly black sulfur staining that would otherwise occur on the interior of uncoated or standard-lacquer-coated cans when sulfur-releasing foods are retort-sterilized.

3) Polyester Lacquer (PE Lacquer):
Single-component thermosetting polyester resin lacquer for food applications requiring high flexibility (deep-drawn can components) or high-temperature performance. Better flexibility than epoxy phenolic during can body forming and neck/flange operations. Used for draw-and-redraw (DRD) can manufacturing where the lacquer must withstand post-lacquer-application forming operations without cracking. Also used for beer and beverage can internal lacquer where neutral taste and odor are critical.

4) Organosol / Vinyl Lacquer:
Plasticized vinyl (PVC-based) lacquer providing excellent flexibility for deep-drawn and wall-ironed (DWI) can applications. Historically used for beverage can internal coating but being phased out in some markets due to plasticizer migration concerns. Still used in certain Asian markets for aerosol can internal lining.

Effect of Internal Lacquer on Tinplate Specification:
When internal lacquer is applied: lower tin coating weight (E2.8 or E1.5 on food-contact surface) can be used because the lacquer provides primary barrier protection, reducing tin usage and cost. The tinplate surface must be correctly prepared for lacquer adhesion — Type 300 cathodic chromate passivation (1-7 mg/m² Cr) is the correct passivation for lacquered tinplate, providing superior lacquer adhesion versus dichromate passivation (Type 311). Oil type must be compatible with lacquer system — ATBC oil (acetyl tributyl citrate) is preferred over DOS oil (dioctyl sebacate) for lacquered applications as it provides better lacquer adhesion and has food contact approval in all major regulatory jurisdictions. Tanglu Group specifies Type 300 passivation and ATBC oil as standard for all food-grade tinplate unless customer specifically requests alternatives.

Incoming quality control (IQC) of tinplate sheet at can-making plants is critical because tinplate quality variations — in hardness, tin coating weight, surface quality, flatness, or dimensions — directly cause costly production line stoppages, defective can production, and potential food safety non-compliance if under-coated or out-of-specification tinplate is processed into food cans. Establishing a rigorous IQC protocol for tinplate receiving inspection protects can-making efficiency and product quality.

Primary Quality Parameters for Tinplate IQC:

1) Hardness Verification (HR30T):
Method: Rockwell 30T superficial hardness test using a diamond indenter, 30 kg total load per EN ISO 6508. Frequency: minimum 5 test locations per sheet (corners + center) on 2 sheets per coil/pallet. Accept/reject criteria: mean HR30T value within specified temper range ± 3 HR30T (e.g., T3: 53-59 HR30T). Reject actions: out-of-temper tinplate — too soft causes buckling and poor can panel rigidity; too hard causes cracking during forming operations and excessive springback. HR30T is preferred over Vickers or Rockwell B for tinplate because the shallow indentation depth (less than 0.10mm total) minimizes through-thickness influence of the thin tinplate substrate on hardness reading accuracy.

2) Tin Coating Weight (XRF Measurement):
Method: X-ray fluorescence (XRF) coating weight gauge — portable handheld XRF units (e.g., Oxford Instruments, Fischer FISCHERSCOPE) or benchtop XRF can measure tin coating weight in g/m² non-destructively in 5-10 seconds per measurement. Frequency: minimum 5 measurement locations per surface (both surfaces) on sample sheets from each pallet/coil. Accept/reject criteria: minimum average coating weight ≥ specified E-class value per EN 10202 Table 6, with no single measurement below 80% of nominal (spot measurement minimum). Reject action: under-coated tinplate risks tin dissolution exceeding food safety limits during product shelf life and inadequate corrosion protection. Differential coating orientation must also be verified (confirm which surface carries the higher coating weight and that it will be the food-contact inner surface in the formed can).

3) Surface Quality Inspection:
Method: Visual inspection under 300-500 lux diffuse lighting and 10-20× magnification for suspected defects. Critical defect types: tin pinholes (bare steel spots where tin coating is absent — verified by wetting surface with 1% HCl solution which turns blue with copper sulfate indicator on exposed steel), surface streaks and roll marks (visible linear marks indicating die line or roll surface defects), splash marks (irregular matte areas from cooling water or plating solution splash during production), reflow irregularities (frosted or non-bright areas indicating incomplete tin reflow), and mechanical damage (scratches, dents, buckled corners from handling). Accept/reject criteria per EN 10202 surface quality classification (standard commercial quality vs. best commercial quality for decorative applications).

4) Dimensional Verification:
Thickness: Micrometer measurement at 5 positions per sheet (edges + center). Accept/reject: within EN 10202 thickness tolerance (±0.010mm for standard tinplate). Width and length: Steel tape or sheet dimension gauge. Flatness: Place sheet on flat reference surface and measure maximum gap — should not exceed 5mm/m for standard can-making quality. Camber (lengthwise curvature): Maximum 3mm in 2,000mm length for coil-fed applications.

5) Passivation and Oil Verification:
Passivation type verification: Simple water contact angle test — correctly passivated (Type 300 cathodic chromate) tinplate shows water contact angle 30-60° (hydrophilic, good lacquer wetting). Inadequate passivation shows higher contact angle. Oil presence: Wipe white tissue across surface — some oil transfer indicates oil is present. Excessive oil (sticky, pooled) indicates over-oiling requiring cleaning before lacquer application.

6) Certificate Verification:
Verify mill test certificate (EN 10204 3.1) against received material: heat number on certificate matches heat number on pallet/coil label, all specification parameters (temper, coating class, passivation, oil type, dimensions) match purchase order and customer specification, food contact compliance declaration matches intended food product application, certificate signatory is authorized mill quality representative. Archive certificates for minimum 3 years (or as required by food safety management system — FSSC 22000, SQF, BRC — audit requirements). Tanglu Group provides pre-shipment quality verification reports and can coordinate third-party inspection at mill for critical shipments requiring enhanced confidence in specification compliance before shipment.