ABS and DNV Classification for Marine Stainless Steel: Grade Approval Process

ABS and DNV both require stainless steel to pass through a two-layer approval system - manufacturer (mill) approval and grade/product approval - before it can be used on a classed vessel. The two societies follow parallel but independently administered processes: an ABS approval does not automatically transfer to DNV, and vice versa, even for the identical alloy and mill.

Stainless steel used below the waterline, in seawater piping, or in critical machinery has to survive decades of chloride exposure without cracking or losing structural integrity. Classification societies exist to verify, independently of the shipyard and the mill, that a given batch of steel actually performs the way its data sheet claims. This article explains how ABS and DNV structure that verification for stainless steel, what it costs manufacturers in time and testing, and what buyers should check before specifying a grade.

What Do ABS and DNV Classification Actually Certify?

Classification certifies that a vessel - and the materials built into it - meets an independent technical standard for structural and mechanical integrity, separate from, but often referenced by, flag-state safety regulation.

The American Bureau of Shipping (ABS) and DNV (Det Norske Veritas, formerly DNV GL) are classification societies: private technical organizations that publish their own rule sets and verify compliance through document review, surveys, and testing. Classification is not the same as flag-state or port-state regulatory approval, but insurers, shipowners, and regulators widely rely on class certificates as evidence that a vessel and its components were built to a recognized engineering standard. For stainless steel components, this means the alloy, its manufacturing process, and its mechanical properties must be traceable to an approved source and verified against the applicable class rules before the material can go into an ABS-classed or DNV-classed vessel.

Are ABS and DNV Requirements the Same for Stainless Steel?

No. Both societies test similar properties - chemical composition, tensile strength, corrosion resistance - but each maintains its own rule book, its own approval categories, and its own surveyors, so approval is not interchangeable.

ABS publishes its stainless steel requirements primarily within its Rules for Materials and Welding, while DNV publishes equivalent requirements in DNV-RU-SHIP Part 2, Chapter 2 (Metallic Materials). Both rule sets converge on similar technical targets - because they are both grounded in recognized international practice - but the administrative pathway differs: separate applications, separate surveyor audits, and separate certificates. A stainless steel mill that wants to supply both ABS-classed and DNV-classed shipyards typically has to complete both approval processes independently, even though much of the underlying test data can be reused across applications.

Which Stainless Steel Grades Are Commonly Used in Marine Applications?

Austenitic grades like 316/316L cover general seawater-contact hardware, while duplex and super-duplex grades are specified where higher strength and superior chloride pitting resistance are required, such as propeller shafts and seawater piping systems.

Grade selection is driven primarily by chloride exposure and required strength: the higher the seawater temperature and chloride concentration, the more likely a project moves from standard austenitic grades toward duplex or super-duplex alloys, which resist localized pitting and crevice corrosion far better than 316L alone.

How Does the Manufacturer Approval Process Work?

A mill must pass an engineering review of its production process, submit test data, and undergo an on-site audit by a class surveyor before any of its stainless steel can be certified for classed vessels.

Both ABS and DNV structure manufacturer approval around the same core sequence:

Application and documentation review: the manufacturer submits its production process description, quality management system, and proposed grade specifications for engineering review.

Supervised test program: a surveyor witnesses production of qualification heats or batches and reviews the resulting mechanical and chemical test data against the applicable rule requirements.

Plant audit: the classification society conducts an on-site audit of the manufacturing facility, verifying process control, traceability systems, and testing equipment calibration.

Approval issuance: once the mill demonstrates consistent, repeatable compliance, the society issues a manufacturer approval certificate, typically scoped to specific grades, product forms, and size ranges.

Ongoing surveillance: periodic re-audits and, in many cases, enrollment in a quality assurance or approved-manufacturer survey program maintain the approval and can reduce the need for a surveyor to attend every production run.

What Documentation Should Accompany Class-Approved Stainless Steel?

Class-approved marine stainless steel should ship with an EN 10204 Type 3.2 certificate for critical applications - meaning an independent class surveyor, not just the manufacturer, has witnessed and validated the test results.

Buyers should always confirm which certificate type the applicable class rule or project specification actually requires - over-specifying 3.2 across the board adds cost and lead time, while under-specifying it on a critical component can cause a shipment to be rejected at the shipyard's incoming inspection.

How Does Product (Grade) Approval Differ From Manufacturer Approval?

Manufacturer approval qualifies the mill and its process; grade or product approval qualifies a specific alloy, product form, and size range produced by that already-approved mill - both are required together.

Manufacturer approval answers the question, "Can this facility reliably produce class-quality material at all?" Grade approval answers a narrower question: "Is this specific alloy, in this specific product form (plate, bar, forging, casting, pipe) and this specific size range, qualified for this specific class rule?" A mill can be an approved ABS manufacturer generally while still needing to add a new grade - for example, a super duplex alloy it has not previously supplied to class-approved projects - to its approved scope before that grade can be certified for a classed vessel. Buyers should check the manufacturer's approval certificate scope line by line, not just confirm that the company holds "ABS approval" or "DNV approval" in general terms.

What Testing Is Required to Qualify a Stainless Steel Grade?

Qualification testing covers chemical composition, tensile and impact properties, and - for many marine-critical applications - corrosion resistance testing such as pitting and intergranular corrosion checks.

Chemical analysis: ladle and product analysis confirming the alloy falls within the specified composition range for the grade.

Tensile testing: yield strength, ultimate tensile strength, and elongation, verified against the class rule's minimum requirements.

Impact testing: Charpy V-notch testing at specified temperatures, particularly relevant for structural and low-temperature service.

Corrosion testing: pitting resistance (e.g., ASTM G48-type testing) and intergranular corrosion testing for grades intended for aggressive seawater or welded service.

Non-destructive testing: ultrasonic or radiographic examination for forgings, castings, and thick-section products, confirming internal soundness.

For duplex and super-duplex grades specifically, ferrite content and phase balance are also checked, since an incorrect austenite-to-ferrite ratio undermines both the strength and corrosion-resistance advantages the alloy is chosen for in the first place.

How Long Does Approval Take, and How Is It Maintained?

Initial manufacturer and grade approval typically takes several months to complete, driven mainly by test program scheduling and surveyor availability; maintaining approval then requires periodic audits rather than a one-time certificate.

The exact timeline depends on how much qualification data the mill already has on file and how many grades and product forms are being added to its scope simultaneously. Once granted, approval is not permanent: both ABS and DNV require periodic re-audits, generally on an annual or multi-year cycle, to confirm the manufacturer's process controls and quality system remain consistent with the original approval basis. A lapsed or suspended manufacturer approval means new material cannot be certified as class-approved, even if the underlying metallurgy is unchanged - which is why buyers should verify a supplier's approval status is current at the time of order, not just at some point in the past.

What Should Buyers Verify Before Ordering ABS/DNV-Approved Stainless Steel?

Confirm the manufacturer's approval scope covers the exact grade, product form, and size; confirm the certificate type matches the application's criticality; and confirm the approval is currently active, not expired.

Scope match: does the manufacturer's approval certificate explicitly list the grade (e.g., S32750), product form (bar, forging, pipe), and size range being ordered?

Certificate type: does the project or class rule call for EN 10204 Type 3.1 or 3.2, and will the supplier actually provide a surveyor-witnessed 3.2 certificate if required?

Current status: is the manufacturer's approval active, or has it lapsed since the last renewal cycle?

Traceability: can heat numbers on the finished product be traced back to the original cast and qualification test data?

Class society match: is the approval held with the specific society (ABS or DNV) that will class the vessel - not merely 'class approved' in general terms?

Frequently Asked Questions

Is DNV approval accepted by ABS, or vice versa?

Not automatically. Each society administers its own manufacturer and grade approval process, so a mill typically needs separate approvals from each society it wants to supply, even for identical alloys.

What certificate type do I need for a propeller shaft forging?

Critical, safety-related components such as propeller shafting typically require an EN 10204 Type 3.2 certificate, meaning an independent class surveyor - not just the manufacturer - has witnessed the required tests.

Can a non-approved mill still supply stainless steel for a classed vessel?

Generally no, when the applicable class rule mandates approved-manufacturer material. Unapproved material can sometimes be accepted through a case-by-case engineering review, but this is the exception, not the standard path, and adds project risk and delay.

What's the difference between 316L and duplex 2205 for marine use?

316L is a general-purpose austenitic stainless steel suited to moderate chloride exposure; duplex 2205 offers roughly twice the strength and significantly better resistance to chloride pitting and stress corrosion cracking, making it the preferred choice for more aggressive seawater service.

Does class approval guarantee the material is corrosion-proof?

No. Class approval verifies the material meets defined mechanical and compositional requirements under the tested conditions; it does not guarantee performance against every possible service environment, so grade selection still has to match the specific application's chloride, temperature, and stress conditions.