Industry News

Screws rusting, stripping or breaking? Common failure analysis and prevention

2026-08-22

Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.

Rust: the most common failure

Rust usually starts with humidity or salt spray combined with damaged plating. Zinc-plated carbon steel often shows red rust within 72–96 hours of salt-spray testing (e.g. ASTM B117) in coastal outdoor use. Switch to stainless 304 / 316 or Dacromet coating to fix it at the material and finish level. Diagnosis follows three questions. Where is the rust? If it appears on a thread flank or along a scratch, the coating was mechanically damaged during installation or transport, and the answer is touch-up paint or a different handling method rather than a different fastener. If it appears uniformly over the whole part, the specification was too thin for the environment, which is a selection issue. If it appears only under the washer or at a joint interface, the cause is crevice corrosion or trapped water rather than the coating. How fast did it appear? Rust within weeks points to a coating defect or contamination; rust after two or three years in a coastal atmosphere may simply be normal depletion of a coating that was never intended for that exposure. What is the environment? A sheltered indoor joint and an exposed rooftop fixing in the same building experience very different chloride loads, and applying one specification to both is a common and expensive mistake. The repair approach should match the diagnosis: coating damage on an otherwise adequate part can be touched up with a zinc-rich spray, but a part whose coating class was never suited to the environment should be replaced with stainless or Dacromet.

Stripping: mostly selection and installation

Stripping is common when a carbon steel screw meets a softer part (e.g. aluminium), when thread pitch mismatches, or when torque is too high. The mechanism is worth separating, because the fix differs. Thread stripping of the internal thread happens when the nut or tapped hole is the weaker element — a common outcome of pairing a high-grade bolt with a low-grade nut, since a 4.8 nut on an 8.8 bolt is the element that will fail. Thread stripping of the fastener itself happens when the screw is driven into an undersized pilot hole, when a self-tapping screw is used in a hole that is too small for its diameter, or when the material is hard and the screw is soft. Head recess stripping, frequently reported as the screw stripping, is actually a drive problem: a worn bit, an undersized key, too much axial force or an off-axis approach. Cross-threading is a start-angle problem rather than a torque problem, and it is usually caused by starting the screw by hand instead of in a locating fixture. Confirm the thread type (coarse / fine) and hole fit before assembly, and tighten with a torque wrench to specification to greatly reduce stripping. For joints involving different materials, the harder decision is often which element should be the replaceable one — it is usually better to have the screw strip than the tapped hole in an expensive casting, which argues for a controlled torque and a deliberately weaker fastener.

Fracture: strength grade, fatigue and hydrogen embrittlement

Fracture mainly results from overload, fatigue or hydrogen embrittlement. The three causes look similar in the field but have different signatures. Overload fracture shows a necked or ductile appearance, often with a visible twist, and points to a genuine capacity problem or a torque far above specification. Fatigue fracture shows a smooth, progressive crack zone with a rough final rupture area, and points to a vibrating joint, insufficient preload that lets the bolt see cyclic load, or a design that places the joint in bending. Hydrogen embrittlement shows a brittle, intergranular failure that occurs hours or days after tightening without any increase in load, and it is specific to high-strength parts that have been acid-pickled or electroplated — 10.9 and particularly 12.9 grade are the at-risk classes, and the risk rises with hardness. Zinc-plated 12.9 grade high-strength bolts carry an embrittlement risk; for demanding jobs choose Dacromet or phosphating. Match grades 8.8 / 10.9 / 12.9 to the real load, and note that moving to a higher grade is not automatically safer — a 12.9 bolt brings a higher torque requirement, greater sensitivity to hydrogen and, in some joints, a brittleness that a correctly preloaded 10.9 would have handled without difficulty. Where a fracture has occurred, the useful evidence is the fracture surface, the grade marking, the coating, and the tightening torque recorded at installation.

Diagnostic table, prevention and acceptance points

A diagnostic sequence by symptom makes field analysis much faster. Rust appearing within weeks, uniformly — coating class too thin for the environment; specify stainless or Dacromet. Rust appearing from a scratch or thread flank — installation or transport damage; add protection at handling and touch up with zinc-rich paint. Rust under the washer only — crevice corrosion; improve drainage, add a sealing washer or raise the grade. Nut thread stripped — grade mismatch between bolt and nut; match or exceed the nut grade. Screw thread stripped in a soft or thin material — wrong screw type or oversized hole; use a thread-forming screw sized to the material, or switch to a rivet nut. Recess rounded — worn bit, wrong key size or excessive axial force; replace bits on a fixed cycle and train the assembly method. Fracture shortly after tightening with no load increase — hydrogen embrittlement; change the coating for high-strength grades and require de-embrittlement baking. Fracture after months of service in a vibrating joint — fatigue; raise the preload, add a proper locking method or redesign the joint. At acceptance, check the grade marking and material certificate against the specification, verify the coating class and thickness, confirm that washers and nuts are matched to the bolt grade, and record the tightening torque on critical joints. Retaining one failed sample with its lot number is what turns an argument into an answer. Need screw selection, a failure analysed or anti-rust advice? Contact JIALI Industrial (164.hk) with your condition and material needs.

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