302 Spring Wire - Hydrogen Embrittlement After Plating

Stainless spring wire is chosen partly because it does not need a coating to resist corrosion. When a part is nevertheless plated, for appearance, solderability or a specific wear requirement, a new failure mode appears: hydrogen embrittlement. The hydrogen enters during pickling and during the plating itself, migrates to the regions of highest stress, and can cause a spring to break days after it was installed and passed inspection. This page explains where the hydrogen comes from, why high strength wire is more vulnerable, and what the process needs to look like to avoid it.

Product Specifications

Where Hydrogen Enters the Process

Pickling and Electroplating

Two steps introduce hydrogen. Acid pickling before plating generates hydrogen at the metal surface as the oxide is dissolved. Electroplating generates it again, because part of the current that should deposit metal instead reduces hydrogen ions at the cathode, and the steel is the cathode. The amount absorbed depends on current density, bath chemistry, plating time and surface condition. Barrel plating, common for small springs, exposes parts to intermittent contact and current spikes and is generally harsher than rack plating.

Why High Strength Wire Is Vulnerable

Sensitivity to hydrogen rises sharply with strength. Above roughly 1000 MPa tensile strength the risk becomes real, and spring temper 302 at 1400-1800 MPa sits well inside the sensitive range. The mechanism is straightforward: hydrogen diffuses into the lattice, collects at stress concentrations, and reduces the local cohesive strength until a crack starts. The inside of a coil and the root of the first active coil are where the stress is highest, and that is where the fracture usually appears.

Recognising and Preventing Failure

Baking Before and After Plating

The standard countermeasure is a bake. De-embrittlement baking at 200-230 °C for four hours or more, started within a few hours of plating, allows hydrogen to diffuse out before it collects at a critical site. Baking before plating, after pickling, removes part of the load earlier in the sequence. The bake has to be specified with a time window from the end of plating, because a bake that begins the next day is much less effective than one that begins within the hour.

Mechanical Alternatives to Electroplating

Where the coating is decorative or for wear rather than for corrosion, mechanical plating, zinc flake coatings and physical vapour deposition introduce little or no hydrogen because they are not electrochemical cathodic processes. For a stainless spring these options are usually worth evaluating, since the substrate already provides the corrosion resistance and the coating only has to survive handling and appearance requirements.

Reading the Specification Correctly

What the Temper Tells You

A spring temper wire at 1800 MPa is a different risk category from a light drawn wire at 900 MPa. If the design allows a lower tensile strength, choosing it reduces embrittlement sensitivity without changing the geometry. Where the higher strength is required for load, the plating and baking sequence has to be controlled rather than assumed.

Test Evidence to Ask For

Process control for hydrogen embrittlement is usually demonstrated by a sustained load test on finished parts, held for a defined period at a stress below the yield point, rather than by a certificate on the wire. Ask the plater for the bake record, with the interval between plating and baking, and for the sustained load test result on a sample from the same lot.

Applications

Springs for Wet Environments

Springs used in marine, food processing and outdoor equipment are often left unplated for exactly this reason. Bare 302 or 316 spring temper wire resists the environment without introducing a plating step, and the elimination of the coating also removes the risk of coating damage in service.

Fasteners and Clips

Small plated clips and retaining rings combine high strength with a large plated area relative to mass, which is the worst combination for hydrogen pickup. Where plating is unavoidable, specify the bake window in the purchase order rather than leaving it to the plating shop.

Electronics and Instrument Parts

Contacts and instrument springs are frequently plated for solderability or conductivity. These parts are usually small, high strength and loaded in service, so the full sequence from pickling through bake needs to be defined as one process rather than as separate purchases.

Product Advantages

• Spring temper 302 with tensile strength controlled by size, so the embrittlement risk category is known before plating is arranged.
• Bright drawn surface free of seams, since a surface defect both starts fatigue and provides a path for hydrogen entry.
• Diameter tolerance of ±0.01 mm on fine sizes, keeping the finished coil stress within the designed range.
• Consistent cast and helix, so the spring does not carry unintended pre-stress from a badly formed coil.
• Test results recorded against the heat number, supporting traceability when a plating related failure is investigated.

Quality Control

Tensile strength, elongation and diameter are tested on every lot. Surface is inspected for seams and laps over the full coil length. Coil set is checked against the agreed value. Where a customer process includes plating, the recommended bake window and the maximum tensile strength for the application are stated on the certificate so that the plating shop has the information at the time it needs it.

Selection and Ordering Notes

1. State whether the finished part will be plated, and by which process. Electroplating and mechanical plating call for different precautions at different stages.
2. If plating is required, write the bake window into the order: temperature range, minimum duration, and the maximum interval between plating and baking.
3. Consider whether a lower temper would meet the load requirement. Reducing tensile strength is the most effective single measure against embrittlement.
4. For unplated springs, confirm the service environment. If chlorides are present, a 316 grade should be evaluated rather than relying on 302.
5. Ask for the sustained load test result on finished parts where the application is safety related, rather than accepting a wire certificate alone.

Hydrogen embrittlement is a process problem rather than a material problem, and it is solved by defining the sequence from pickling to bake as one controlled operation. Tell us the plating process and the load the spring must hold, and the wire temper and the post-plating requirements can be matched to both.