Hydrogen Embrittlement in High-Strength Marine Fasteners: Why Plating Can Crack the Bolt

A high-strength bolt in a ship's flange joint can fail silently hours or days after installation, with no visible deformation and well below its yield strength. When that happens on electroplated or acid-pickled parts, the usual culprit is hydrogen embrittlement (HE). This article explains how hydrogen enters marine fasteners during plating, which strength grades are genuinely at risk, and how baking and testing per published standards prevent delayed fracture.

What Is Hydrogen Embrittlement?

Hydrogen embrittlement is a permanent loss of ductility in a metal caused by the combined action of mobile (diffusible) hydrogen and applied or residual tensile stress (per the definition recognized in ASTM F2078 and explained in ISO/TR 20491). It is not a slow corrosion; it is a delayed, brittle failure that usually requires only three conditions:

• a steel fastener that is inherently susceptible to HE (higher strength = higher susceptibility);
• the presence of mobile hydrogen in the steel;
• a significant tensile stress, either applied in service or residual from manufacture.

When atomic hydrogen enters the steel, it causes cracking at stresses far below the yield point — typically described as hydrogen-induced delayed brittle failure (see the scope note in ISO 15330). Because the part does not stretch or neck before breaking, the failure looks sudden and unexplained at the shipboard level.

Where Hydrogen Comes From During Plating

The most common source of internal hydrogen in electroplated marine fasteners is the production process itself. Hydrogen can be introduced during acid cleaning or pickling, phosphating, and electrolytic plating (which generates hydrogen at the cathode), as well as during heat treatment and some machining operations (based on the failure-mechanism background in ISO 15330:1999 and ISO/TR 20491).

• Acid pickling / acid cleaning of the surface before coating — a standard pre-treatment that can dissolve hydrogen into the substrate.
• Electrolytic (electroplated) coating such as zinc electroplating — hydrogen is evolved at the cathode and can be partially absorbed by the steel.
• Phosphating and some chemical surface treatments.

On board, additional hydrogen can be generated in service by corrosion reactions or by cathodic protection; this is usually referred to as environmental hydrogen embrittlement (EHE), as opposed to internal hydrogen embrittlement (IHE) from plating. Both are considered in the fastener hydrogen-embrittlement literature referenced by ISO/TR 20491.

Which Fasteners Are at Risk

The deciding factor is material strength and hardness, not thread size. Low- and medium-strength grades such as ISO property class 4.8 and 8.8 have little susceptibility to IHE, which is why galvanized carbon-steel cargo fasteners in these grades are routinely supplied without special hydrogen controls. Susceptibility rises sharply with hardness above roughly 390 HV.

Mechanical values and hardness ranges for ISO 898-1 property classes are from ISO 898-1; the corresponding hydrogen-embrittlement measures are set out in ISO 4042:2022 clause 4.4 and Table 3. Note that ISO 4042:2022 identifies fasteners with a specified minimum hardness above 390 HV as susceptible fasteners requiring mandatory baking — the same threshold used by ASTM F1941/F1941M (above 39 HRC). This matters because older, pre-2018 practice often recommended baking for grades down to 320 HV (e.g. 10.9) as a precaution; current research referenced by ISO/TR 20491 indicates fasteners below about 390 HV do not show significant IHE sensitivity, provided manufacturing processes are well controlled.

How Platers and Specifiers Prevent It

• Controlled pre-treatment (ISO 9587): minimize acid pickling time and concentration; use inhibited acids where possible to reduce hydrogen uptake before coating.
• Post-plating baking (ISO 9588): a heat treatment applied after electroplating so that mobile hydrogen diffuses out of the steel. Baking is typically performed at about 200 °C (applied within the baking temperature guidance referenced in ISO 9588 and coating-process specifications); duration and temperature are selected in accordance with the applicable coating and strength requirements.
• Low-hydrogen coating processes: where hydrogen exposure is critical, specifiers may select processes such as alkaline zinc-nickel (12–16% Ni), recognized as low-hydrogen-embrittlement (LHE) processes; ISO 4042:2022 clause 4.4.4 notes these may allow baking to be avoided, subject to testing and agreement between supplier and purchaser.
• Strength-grade selection: for new designs, avoid unnecessary high hardness where a 10.9 or 4.8/8.8 grade meets the design load, since lower grades are far less susceptible.

Verification and Testing

Because hydrogen embrittlement is a delayed failure, it must be verified by test, not by visual inspection. Common in-process methods include:

• ISO 15330 (in China adopted as GB/T 3098.17-2000): a preloading test on parallel bearing surfaces that detects internal hydrogen embrittlement in steel fasteners under tensile load. It is intended for in-process control — the chance of detecting HE decreases significantly if the test starts more than 24 h after the last manufacturing step (see ISO 15330 scope note).
• ASTM F519: mechanical hydrogen-embrittlement evaluation of plating/coating processes, using sustained load over a defined period.
• ASTM F1941: as well as specifying electrodeposited coatings, it identifies susceptible fasteners (above 39 HRC) that require baking and confirms baking as a mandatory requirement for those coatings unless otherwise agreed.

Procedural guidance on the causes, control, and testing of HE in steel fasteners is consolidated in ISO/TR 20491, Fasteners — Fundamentals of hydrogen embrittlement in steel fasteners. These tests do not relieve the manufacturer of implementing and monitoring sound process control (ISO 15330).

Standards Reference

FAQ

Q: Do galvanized 4.8/8.8 carbon-steel bolts in cargo holds have hydrogen-embrittlement risk?

These low- and medium-strength grades have no significant internal-hydrogen-embrittlement sensitivity, so special baking is not generally mandated for them (ISO 4042:2022 clause 4.4.2). The risk concentrates in higher-hardness, quenched-and-tempered grades.

Q: Why is a 10.9 or 12.9 bolt at risk but a 4.8 bolt of the same size is not?

Susceptibility to HE is governed by material hardness and strength, not thread size. Quenched-and-tempered high-grade steel (e.g. above 390 HV) traps mobile hydrogen far more effectively and fails in a brittle, delayed manner; low-grade steel tolerates the same hydrogen without losing strength.

Q: Is baking always required after electroplating?

No. Under ISO 4042:2022, mandatory baking applies to fasteners with specified minimum hardness above 390 HV (clause 4.4.4); for grades such as 10.9 the standard requires process/product verification and/or baking depending on the measures selected (clause 4.4.5, Table 3). ASTM F1941 similarly requires baking above 39 HRC.

Q: What test proves a plated lot is free from hydrogen embrittlement?

For in-process control, ISO 15330 (GB/T 3098.17-2000) preloads fasteners on parallel bearing surfaces to detect HE; it should be started within 24 h of the last manufacturing step to be effective. ASTM F519 is another recognized sustained-load method for evaluating plating processes.