Why does a plated steel rod end rust?
Rust is iron oxidising in the presence of water and oxygen. Salt and road chemicals make it worse, because they keep the wet film conductive, so the reaction runs faster. A coating does not change the steel underneath. It only stops water and oxygen reaching the steel, and it does that only while it is whole.
That is why the coating type matters. A barrier coating, such as nickel, chrome or paint, works by sitting on top of the steel. Where it is scratched through, the steel is exposed and rusts. Zinc works differently. Zinc is more active than steel, so at a scratch it corrodes first and protects the steel next to it. That is called sacrificial protection, and it is why zinc tolerates small damage better than a barrier coating, but it also means the zinc is used up over time.
What do the common choices give you?
| Material or finish | Where it helps | Where it falls short |
|---|---|---|
| Carbon steel, unplated | High load capacity at low cost | Rusts without protection. Not suitable for wet or corrosive places |
| Carbon steel, zinc-plated | Economical protection for mild exposure. A chromate layer improves it | Sacrificial zinc is used up in constant wet or salt. Abrasion wears it off |
| Carbon steel, nickel or chrome-plated | Better corrosion resistance and a finished look. Costs more | A barrier only, so any scratch to the steel rusts |
| Alloy steel (chromoly) | Stronger than carbon steel, moderate corrosion resistance. Usually plated for outdoor use | Moderate resistance is not enough for constant wet or salt |
| Stainless (304 or 316) | Corrosion resistant without a coating. 316 does better in salt water | Costs more, slightly lower load capacity, and can gall on the thread (see below) |
| Polymer or composite housing | Light, does not rust, often maintenance-free with a self-lubricating liner | Lower strength than metal. Must be matched carefully to the load |
Black oxide is a cosmetic and mildly protective finish, mainly for indoor use. It needs oiling to protect at all.
Why does stainless resist rust, and when does it fail?
Stainless steel resists rust because a very thin oxide film forms on its surface and repairs itself when it is damaged, as long as oxygen is available. That is the whole mechanism. The film can be broken locally by chlorides from salt, and by crevices, such as the space under a washer, inside a tapped hole or at a bolt head, where oxygen cannot reach to repair it. A stainless part in a crevice full of salt water can corrode even though the open surface looks clean.
So stainless is not "rust-proof". It is the better choice for wet and washdown service, but the design still has to avoid trapped water and salt. Drain the water, avoid crevices where you can, and rinse after washing.
What makes a coating fail in the field?
The problems are mechanical as well as chemical. On farm equipment, mud, fertiliser and road salt promote corrosion, and regular impact and abrasion wear down protective layers. The same is true of any machine that is washed down, knocked or dragged through grit. Once the finish is scraped or worn through, the steel underneath is exposed, and the coating stops protecting the joint.
Grease and oil work as a barrier in service too, and where a greased joint is washed, the grease can be stripped out and the steel exposed. That is the same washout problem covered in Q1-08.
What problems come with stainless?
Two problems come with stainless, and both apply to a threaded shank.
Galling is thread seizure. Two metal surfaces under pressure and movement can stick and tear, which is common in stainless steel, aluminium and titanium. It is rare in carbon and alloy steel. It happens during tightening, often within seconds. The most dangerous pairing is a stainless fastener running in a stainless tapped hole. Use an anti-seize compound or a different material for the mating thread where galling is likely.
Galvanic corrosion happens when two different metals touch in the presence of moisture. The less noble metal corrodes faster, and the further apart the two metals are on the galvanic series, the faster the attack. The practical rule is that the fastener should be the more noble material, or the two should sit close together on the series. The metals table is in K05-Q1-09.
A coating on a steel fastener changes how well it stands up to the environment, not whether the two materials are compatible.
How do you decide?
Ask five questions in order.
How wet is the joint? Occasional damp is a different case from spray and washdown.
What is in the water? Salt, fertiliser and cleaning chemicals each attack in different ways. Salt is the one that most often breaks down protective films, so ask about it first.
How much abuse does it take? Mud, grit and knocks wear coatings off, and a worn coating is worse than none in a crevice.
How big is the load? Check the load rating of the stainless or polymer option against the real load (Q1-02). Do not assume a stainless part is strong enough because it is corrosion-proof.
What does it screw into? Check the galling risk of the threads and the galvanic pairing with the mating part.
If you are choosing a linkage rod and adjuster, the material choices are covered in K06-Q1-08.
How do mud, salt and impact change the material and finish choice?
An off-road vehicle is a washdown machine with grit and impact added. It drives through mud and water, dries, is hosed down, and in many regions runs on salted roads between events. The finish takes stone chips and tool marks as well as the chemistry, so the "what makes a coating fail" section above describes it well. A few vehicle points are worth adding.
Strength and finish pull in different directions. Suspension and steering links are often built on alloy-steel rod ends, because stainless gives up some load capacity. That puts the weight on the finish and on the routine: rinse mud and salt off, dry the joint, check the plating for chips and rust at the shank and under the jam nut, and renew the grease if the wash removed it (Q1-08). Boots and covers keep grit out but also hold wet mud against the joint if they are not cleaned.
Hardened, plated parts need a question. Electroplating can introduce hydrogen into very hard steel, which can embrittle it unless the maker controls the process, usually with a bake after plating (a fastener-industry paper on hydrogen embrittlement explains the mechanism). Ask the supplier how plated high-strength parts are handled.
Watch the neighbours. Aluminium brackets and spacers next to steel or stainless set up the galvanic pairing described above, and spacer and washer faces form crevices that hold salt water (K05-Q1-09).
For the vehicle application see off-road rod ends; for materials see chromoly rod ends and stainless rod ends. The material and finish of a steering or suspension link are the vehicle designer's decision.