302 Spring Wire - Why Stainless Wire Shows Rust Spots

Every few months a customer reports that the stainless wire has arrived rusty, and almost every time the cause is not the wire. Three different mechanisms produce a brown mark on 302 spring wire, and only one of them is a material problem. Distinguishing them takes a few minutes and decides whether the answer is a handling change, a cleaning step, or a different grade. This page describes the three, how to tell them apart in a workshop, and where the boundary between 302 and 316 actually sits.

Product Specifications

Three Different Problems Called Rust

Free Iron Contamination

Particles of carbon steel settle on the stainless surface and rust there. The particle itself is the problem, not the wire beneath it. Sources are ordinary: a shear blade, a grinding wheel used on both materials, a forklift, an unlined steel bin, a tool left on the coil. In humid air the particle rusts within hours and the stain spreads sideways across the stainless, which is why the mark often looks larger than any visible particle.

Pitting Corrosion

Chlorides break down the passive layer locally and a small pit forms, usually with a brown corrosion product around it. This is a genuine material limitation and it depends on grade. The pitting resistance equivalent number for 302 is around 19 because it contains no molybdenum, while 316 reaches about 27 with 2.5% molybdenum. That gap is the difference between surviving a chloride environment and failing in it.

Tea Staining and Heat Tint

Tea staining is a superficial discoloration that appears on stainless exposed to moisture with surface contamination present; it is cosmetic and does not penetrate. Heat tint is different: the oxide produced by welding or high temperature exposure is chromium depleted underneath and does reduce corrosion resistance unless it is removed and the surface repassivated.

Diagnosing Which One You Have

A Simple Field Test

Wipe the mark with a cloth and a mild acid cleaner. Free iron contamination usually lifts off with the particle and leaves bright metal underneath. Pitting leaves a visible pit that cleaner cannot remove. Tea staining fades but the surface stays discoloured. A ferroxyl test with potassium ferricyanide gives a blue colour where free iron is present and is worth keeping on the shelf for incoming inspection.

What the Pattern Tells You

Marks clustered where the coil rested on a surface, or in a line where a tool touched it, point to contamination. Marks spread evenly over the whole coil, or concentrated in the crevices between coils, point to condensation in transit. Marks only at weld zones point to heat tint and a missing post-weld treatment.

Prevention and Grade Choice

Handling and Cleanliness

Dedicated stainless tools, covered storage, plastic or stainless contact surfaces and dry packing remove most contamination complaints at almost no cost. Where the part will be visible, a citric or nitric passivation step after forming removes embedded iron and restores the passive layer. For transit, desiccant and a sealed liner are more effective than any coating on the wire.

When 302 Is Not Enough

302 is the right choice for indoor parts, dry service and mildly humid environments. It is the wrong choice for coastal air, chloride cleaning agents, swimming pool areas and any application where the part stays wet. In those conditions 316 or 316L is not an upgrade for prestige, it is the minimum requirement, and 302 will fail regardless of how well it is handled.

Applications

Outdoor Springs and Fasteners

Outdoor parts in inland areas with occasional rain usually perform in 302 provided the surface stays clean. Parts that sit in standing water or see road salt should be specified in 316 from the beginning, since no amount of passivation compensates for the missing molybdenum.

Food and Beverage Equipment

Food equipment is cleaned with chloride based sanitisers, which is precisely the environment that pits 302. Springs and wire forms in this sector are normally specified in 316L, with a passivation step after forming and before the first cleaning cycle.

Coastal and Marine Adjacent

Within a few kilometres of the sea, airborne chloride deposits on any surface that is not washed by rain. In these locations 302 parts show pitting within months on sheltered faces, while 316 lasts for years. The distance to the coast and the shelter of the installation are the two facts worth establishing before choosing the grade.

Product Advantages

• Bright drawn surface free of embedded iron, so a rust mark on arrival points to handling rather than to the coil.
• Passivation available to ASTM A967 on request, removing surface contamination before shipment.
• Grade advice based on the actual environment: 302 where it is sufficient, 316 where it is not.
• Diameter tolerance of ±0.01 mm on fine sizes, keeping coil contact and drainage behaviour consistent.
• Soap coating option that protects the surface in transit without sealing in moisture.

Quality Control

Surface is inspected over the full coil length for embedded iron, die marks and stains before packing. Where passivation is specified, the process and the test method are recorded with the lot. Packaging is checked for dry conditions and for contact with carbon steel. Composition is verified per heat so that the chromium and molybdenum content behind the corrosion claim can be confirmed.

Selection and Ordering Notes

1. Describe the service environment, not only the grade: indoor, outdoor, coastal, or regularly cleaned with chlorides.
2. State whether the part will be visible, as cosmetic requirements decide whether passivation is needed.
3. For welded parts, require post-weld cleaning and passivation, since heat tint cannot be avoided at the weld itself.
4. Specify dry packing with desiccant for sea freight, especially for coils that will be stored before use.
5. Ask for a ferroxyl test result on the first delivery if embedded iron has been a problem before.

A rust mark on stainless wire is a message about the environment or the handling, and reading it correctly avoids both unnecessary grade upgrades and repeated complaints. Send the service conditions and the cleaning regime, and the grade and surface treatment can be matched to what the part will actually experience.