What Hydrogen Embrittlement Is: The Signature of Delayed Fracture

Hydrogen embrittlement (HE) is the phenomenon in which hydrogen atoms, after penetrating high-strength steel, gather at internal defects under tensile stress, induce cracks and slowly propagate them, ultimately causing brittle fracture. It has three traits that especially trouble field engineers: first, delayability—fracture does not occur at the moment of loading but hours, days or even months after installation; second, low stress—the actual stress at fracture is far below the material's yield strength and proof load; third, no warning—the fracture surface is flat with almost no plastic deformation, like a "sudden break". Macroscopically the surface is a brittle flat break; microscopically it is intergranular or quasi-cleavage, completely different from the dimpled surface of a normal overload tensile fracture.

Where Hydrogen Comes From: Plating, Pickling and Service Environments

At room temperature steel in air does not absorb hydrogen; hydrogen mainly comes from processes and environments that "force hydrogen out":

• Electro-galvanizing / cadmium plating: cathodic hydrogen evolution is a side reaction of plating; hydrogen atoms deposit with the coating and penetrate the substrate—the number-one source of hydrogen embrittlement in high-strength fasteners.
• Pickling / acid rust removal: acid reacts with iron to evolve large amounts of hydrogen; without inhibitors or with over-long pickling, hydrogen uptake is severe.
• Phosphating, welding, cathodic protection: likewise generate atomic hydrogen at the surface.
• Service environments: dampness, seawater, cathodic-protection potential and hydrogen-sulfide environments (e.g. oil/gas fields) make fasteners continuously absorb hydrogen in use, called "environmental hydrogen embrittlement"; in such cases even bake-out immediately after plating cannot fully protect the joint, and material upgrade or protective coating must be considered from the design stage.

Who Is Most Susceptible: Strength Class and Material Thresholds

The higher the material strength and hardness, the more susceptible it is to hydrogen embrittlement. Engineering broadly takes tensile strength around 1000 MPa and hardness around HRC 32 (about 300 HV) as an important warning line: fasteners below this value are generally not considered at risk; at or above it (e.g. class 10.9 and 12.9, often reaching HRC 39 and above), hydrogen-embrittlement control must be made mandatory. In addition, inclusions, tempered-martensite microstructure, surface decarburization and defects in high-strength alloy steel further raise susceptibility—this is why tooling screws and high-strength bolts, once they take the zinc route, must pair a dehydrogenation step.

GB/T 3098.17: The Parallel-Supporting-Surface Preload Test

GB/T 3098.17-2000 (mechanical properties of fasteners; preload test for the determination of resistance to hydrogen embrittlement; parallel-supporting-surface method, corresponding to ISO 15330) is the core standard for judging the hydrogen-embrittlement susceptibility of steel fasteners. Its logic is direct: clamp the specimen in a dedicated parallel-supporting-surface fixture to a specified preload, hold for a period, and observe whether it cracks or fractures. Because hydrogen embrittlement needs both "tensile stress" and "diffusion time", the preload test applies both at once: the parallel-supporting-surface fixture keeps the thread free of extra bending moment, testing only the material's intrinsic resistance to hydrogen-assisted cracking under constant tensile stress.

During the test, apply a preload corresponding to the product size and standard, hold for the specified time, then unload and inspect—if the specimen cracks or fractures, it is judged susceptible to hydrogen embrittlement and the batch process fails. The method needs no expensive electron microscope and can be batch-sampled on the line, making it the universal means of validating plating/bake-out processes. Emphasize: the preload test is a "process release" tool—it verifies whether "this batch, this process" is safe, not a lifetime verdict on a grade.

Prevention: Bake-Out and Process Control

Since hydrogen enters during plating/pickling, prevention works from both ends: "absorb less hydrogen" at one end and "drive it out" at the other. Bake-out is the most effective engineering means—as soon as possible after plating (generally within a few hours), put parts in an oven at about 190–220°C for a few to twenty-some hours, letting absorbed hydrogen atoms diffuse out. Too high a temperature damages strength; too short a time leaves incomplete dehydrogenation; the process window must be found and locked by strength class and size. Beyond baking, a more thorough route is to replace electro-zinc with zinc-aluminium coatings (Dacromet/Geomet type)—the dip-spin plus sinter process evolves almost no hydrogen and has become the mainstream corrosion protection for NEV and automotive high-strength fasteners; optimizing pickling inhibitors and shortening pickling time also reduces hydrogen uptake at source.

Engineering Checklist: Keeping Hydrogen Embrittlement Out of Assembly

• For any class-10.9-and-above fastener, or one harder than about HRC 32, the corrosion-protection scheme must first assess hydrogen-embrittlement risk.
• For high-strength parts prefer low-hydrogen processes such as zinc-aluminium coating and mechanical galvanizing; if plating is truly required, bake out promptly after plating.
• For critical batches sample the parallel-supporting-surface preload test per GB/T 3098.17, releasing only on pass.
• Avoid over-pickling in assembly and avoid long-term high tensile stress in sulfur-containing/damp environments.