Plated Spring Wire and Delayed Fracture: Hydrogen Embrittlement in 302 Fencing Hardware

A fencing gate latch spring that breaks two days after galvanising, without bending and without wear, is a classic delayed failure. The wire met its tensile specification and the spring worked before plating. What changed is that hydrogen entered the steel during pickling or electroplating, and the coil has been carrying enough stress to pull that hydrogen into the places where cracks start.

Why High Strength Spring Wire Is Vulnerable

Hydrogen embrittlement is a strength-dependent phenomenon. As tensile strength rises, the amount of hydrogen needed to cause cracking falls and the time to failure shortens. Spring temper 302 wire is cold drawn to roughly 1,500 - 2,000 MPa, which places it in the range where plating processes must be controlled rather than merely specified.

Three properties combine: the material is notch sensitive, so any surface defect concentrates stress; the drawn structure has a high dislocation density that traps hydrogen; and a formed spring is already loaded by residual stress from coiling, even before it is installed.

Where the Hydrogen Comes From

• Acid pickling and descaling. Hydrochloric or sulphuric acid used to remove scale or oxide generates hydrogen at the steel surface, and a fraction of it diffuses inward.
• Electroplating. Zinc, nickel and zinc-nickel plating are cathodic processes; hydrogen is evolved at the part being plated, which is exactly the part that matters.
• Cathodic cleaning and electro-cleaning. Often overlooked because it happens before the plating tank.
• Phosphating and chromate conversion. Acid-based pretreatment steps add exposure time even when no current is involved.
• Cathodic electrocoating and corrosion protection by impressed current. A coating process can charge the part, and so can a cathodically protected structure in service.

Note that acid exposure is cumulative. Three short pickling steps in a rework loop can deliver more hydrogen than one long step, because each step recharges the surface layer.

What a Hydrogen Failure Looks Like

The fracture is brittle in appearance: no necking, no shear lip, and a flat or slightly granular face. The origin is normally at or just below the surface, often at a die mark, a tool mark or a corrosion pit. Cracks propagate mainly along grain boundaries in the drawn structure.

The distinguishing feature is time. A hydrogen crack grows subcritically under a static load, so the part fails hours to weeks after processing, usually under a load far below its measured tensile strength. An overload break on the same spring shows deformation and a ductile zone; if the break occurred during assembly, that is the more likely explanation.

De-Embrittlement Baking

The standard remedy is a low temperature bake that lets hydrogen diffuse out before it accumulates at a critical defect. For spring temper stainless wire the working window is narrow:

• Start within a defined interval after plating, commonly within one to four hours and never after the part has been sitting overnight. Hydrogen moves at room temperature, and a delay allows it to reach the trap sites from which it is slow to leave.
• Bake at 190 - 230 C for 4 - 24 hours, with the time and temperature selected by wire strength and coating thickness. Higher strength and thicker deposits need the longer end of the range.
• Do not exceed the tempering or stress-relief temperature of the wire. Cold drawn 302 loses strength above roughly 250 C if held for long periods, so a bake that is hotter than necessary trades one failure mode for another.
• Bake before any bending that loads the part, and bake in an oven with measured uniformity, not in a drying cabinet with an unverified set point.

ASTM B850 provides guidance on post-coating treatments for reducing hydrogen embrittlement risk, and ISO 4042 sets out bake requirements for electroplated fasteners by strength class. Both are useful as a starting framework, but the bake schedule for a specific spring should be validated by test rather than assumed.

Process Routes That Avoid the Problem

For fencing hardware the practical choice is often between a mechanical or flake coating that carries no electrolytic hydrogen risk, and electroplating with a validated bake. A third option, changing the base material to a grade that is inherently more corrosion resistant so that no coating is needed, is worth considering where the spring is visible.

Verification

• ASTM F519 uses a notched, sustained-load specimen to evaluate a plating process for hydrogen embrittlement; it tests the process, not the batch.
• Bend-and-hold test. Form the finished plated spring to a defined deflection and hold for 24 to 96 hours under no further load. Any fracture is a process failure.
• Compression at installed height. Load a sample to its service height and hold it for 96 hours; this reproduces the actual stress state better than a bend test.
• Fracture inspection. Section a failed part and check for intergranular cracking and an origin at a surface defect, rather than assuming a material defect.

What to Write on the Order

Hydrogen embrittlement is controlled by process detail, so the purchase order has to carry process detail: coating method, whether acid pickling is permitted and for how long, the maximum interval between plating and baking, the bake temperature and duration, and the acceptance test with its hold time. A line that reads only galvanised is not enough for a spring temper part.

Wire Specification

Most plated spring failures are not a bad heat of wire. They are a correct wire that was pickled, plated and put into a box without the bake step that would have removed the hydrogen.