MTC vs CoC vs CoA: Understanding the Differences

Three types of quality certificates frequently appear in metals procurement and industrial project documentation: the mill test certificate (MTC), the certificate of conformance (CoC), and the certificate of analysis (CoA). They look superficially similar — all are quality documents from a supplier — but they carry very different levels of evidence and are not interchangeable in most regulated applications.

Quick Answer

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Mill Test Certificate (MTC)

A mill test certificate is the most comprehensive of the three documents. It is issued at the point of manufacture for a specific heat of metal and contains:

• Actual measured values — every chemical element percentage and every mechanical test result recorded for that specific heat
• Heat number — a unique identifier linking the certificate to the physical material
• Specification conformance — a statement that the actual values comply with the applicable standard
• Signatory — the mill's authorised inspector, and (for EN 10204 Type 3.2) an independent third-party inspector

What makes an MTC distinctive is traceability and actual data. The values are not estimated, typical, or representative of the grade — they are the measured results from the specific batch.

Used in: structural steel, pressure vessels, piping systems, oil and gas equipment, power generation, shipbuilding — any application where material properties must be verified and traceable.

For a complete explanation, see What Is a Mill Test Certificate?

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Certificate of Conformance (CoC)

A certificate of conformance is a declaration — a statement by the supplier that the supplied product conforms to specified requirements. It does not contain actual test data.

A typical CoC states something like:

"We certify that the materials supplied under Purchase Order [XXXXX] conform to the requirements of [Standard/Specification]."

Key characteristics:

• No actual test values — the supplier is asserting conformance, not proving it with data
• Traceable to a PO or shipment, not necessarily to a specific heat or lot
• Based on supplier's quality system — the reliability of a CoC depends entirely on the supplier's own testing and documentation processes
• Often used for hardware items — fasteners, gaskets, seals, fittings, and other items where full MTC-level documentation is impractical or not required by specification

When a CoC is acceptable: for non-critical, non-pressure, non-structural applications where the governing specification allows it, or for items that are not metallic (e.g., non-metallic gaskets, sealing materials).

When a CoC is not acceptable: for pressure boundary materials, structural members, or any application where the code or contract mandates actual test data. A CoC cannot substitute for an MTC in these cases.

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Certificate of Analysis (CoA)

A certificate of analysis reports the results of laboratory analysis of a specific lot of material. It is most commonly used in:

• Chemical and pharmaceutical supply chains — for raw materials, excipients, APIs
• Metals in specialty applications — for reporting elemental composition of alloys or metal powders
• Non-ferrous metals — aluminium, copper, titanium, and exotic alloys

For metals, a CoA typically reports:

• Chemical composition (elemental percentages)
• Sometimes physical properties (density, particle size for powders)

What distinguishes a CoA from an MTC in the metals context:

• A CoA usually does not include mechanical properties (tensile, yield, elongation, impact)
• It is often used for incoming inspection of raw material at a processor, not as a primary supply document for end-user qualification
• In some industries, "CoA" and "MTC" are used interchangeably to mean chemical composition data — but this varies significantly by sector

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Side-by-Side Comparison

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Which Document Do You Need?

For pressure-containing equipment

An MTC is required. Most pressure vessel and piping codes (ASME Section VIII, ASME B31.3, PED 2014/68/EU) mandate traceable material documentation with actual test data. A CoC alone is not compliant.

For structural steel

EN 1090 requires EN 10204 Type 3.1 inspection certificates. A CoC does not satisfy this requirement.

For fasteners and hardware

Project specifications vary. Some require MTCs for high-strength fasteners (especially those used in pressure service or in fire-hazardous areas). Others accept a CoC from a recognised manufacturer. Always check the project specification.

For non-metallic items

A CoC from a reputable manufacturer is typically sufficient for gaskets, seals, insulation, and similar items.

For pharmaceutical and food-grade equipment

A CoA for the stainless steel composition is typically required alongside the MTC. Both may be needed to confirm both mechanical fitness and alloy purity.

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Common Confusion and Mistakes

Accepting a CoC in place of an MTC. This is the most dangerous substitution. A buyer who accepts a CoC for pressure boundary material when the specification requires an MTC is accepting a declaration rather than evidence of compliance. If the material later fails, the supplier's declaration provides no technical defence.

Treating a CoA as a full MTC. A CoA that only reports chemistry does not satisfy specifications that also require mechanical property certification. Confirm whether a full MTC or a chemistry-only certificate is acceptable for the application.

Not specifying the required document type on the PO. Purchase orders that say "supply with certificates" without specifying the type leave room for the supplier to provide the least effort document type. Always specify: "EN 10204 Type 3.1 Inspection Certificate" or equivalent.

Accepting a combined CoC/CoA as an MTC. Some suppliers issue documents labelled "Certificate of Conformance and Analysis" that combine a conformance declaration with chemical data. These are not MTCs unless they also include actual mechanical test results from the specific heat.

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Frequently Asked Questions