Which kinds of stainless steel are less likely to rust?

Which kinds of Stainless Steel are less likely to rust?
There are three main factors that influence the corrosion resistance of stainless steel:

1. Alloying element content
In general, when the chromium content of steel reaches about 10.5 %, it begins to exhibit stainless behaviour.  In alloys with higher chromium and nickel content, the corrosion resistance becomes even better. For example, in grade 304 stainless steel the nickel content is about 8-10 % and chromium is about 18-20 %, which under ordinary conditions means such steel will not easily rust. 

2. The smelting / production technology of the manufacturer
Even with the right alloying composition, the producer’s metallurgy, equipment and process control matters. A steel-maker with advanced equipment and good processes will better control alloy element levels, remove impurities, and manage cooling of the steel slab; this leads to more stable, reliable product quality that is less prone to rust. On the other hand, plants with outdated equipment or poor processes may leave impurities, uneven structure or other defects that make rust more likely.

3. External environment — climate and exposure conditions
No matter how good the steel is, the surrounding environment still matters. In a dry, well-ventilated atmosphere corrosion is less likely. In contrast, high humidity, continuous rain, or atmospheres with high acid/alkali/salt content accelerate rusting. Even grade 304 stainless steel can show rusting in very poor environments.

What to do when stainless steel shows surface rust spots?

Chemical method
Use an acid-pickling paste or spray to treat the rusted area and re-passivate the surface so the chromium oxide film reforms and restores corrosion resistance. After acid cleaning you must rinse thoroughly with clean water to remove any acid or contaminants. Then polish the surface and apply a sealing wax to finish. For light rust spots you may use a 1:1 mixture of petrol and machine oil on a clean cloth to wipe off the rust.

Mechanical method
Use sand-blasting, glass/ceramic bead blast, brushing or polishing. Mechanical cleaning may remove contaminants and rust, but it does not change the inherent corrosion resistance of the steel. After mechanical cleaning you should polish and seal the surface. Also it’s best to do the mechanical cleaning in dry conditions, since any lodged foreign iron particles (for example) could act as local corrosion initiators in a humid environment.

Common stainless steel grades and their properties

• 304: One of the most widely used austenitic stainless steels. Good for deep-drawing, acid transport pipes, containers, structural parts, instruments, etc. Also suitable for non-magnetic or low-temperature equipment.

• 304L: A low-carbon version of 304 developed to reduce the risk of grain-boundary carbide precipitation (sensitisation) and thereby improve resistance to intergranular corrosion. Its strength is slightly lower than 304, but its corrosion resistance post-welding is significantly improved.

• 304H: A variant of 304 with higher carbon (0.04–0.10 %) for better high-temperature strength compared to standard 304.

• 316: Based on ~10Cr-18Ni-12 (by weight) steel plus added molybdenum (Mo) so that it has better resistance to reducing media and pitting corrosion than 304, especially in seawater or chloride-rich environments.

• 316L: A low-carbon version of 316 that also improves resistance to sensitisation in welded thick sections and heavy welded equipment, suitable for petrochemical equipment.

• 316H: A high-carbon version of 316 (0.04-0.10 % C) with better high-temperature strength than standard 316.

• 317: Offers better pitting resistance and creep resistance than 316L; used in petrochemical and organic-acid corrosion equipment.

• 321: Titanium-stabilised austenitic stainless steel. The addition of titanium improves resistance to intergranular corrosion and gives good high-temperature mechanical properties, but except for specialised high-temperature or hydrogen-corrosion contexts it is generally not recommended for everyday use.

• 347: Niobium-stabilised austenitic stainless steel. The niobium addition improves resistance to intergranular corrosion and it has similar corrosion resistance in acid/alkali/salt media to grade 321. Good weldability. Widely used in power-plants, petrochemical piping, heat exchangers, furnace tubes.

• 904L: A “super” austenitic stainless steel invented by Outokumpu (Finland). Nickel content is ~24–26 % and carbon <0.02 %. It has excellent corrosion resistance particularly in non-oxidising acids like sulphuric, acetic, formic, phosphoric acids, and good resistance to crevice corrosion and stress corrosion cracking. However, for cost and design reasons, in general applications designers don’t proactively choose 904L unless required.

• 440C: A martensitic stainless steel with the highest hardness achievable among stainless steels (~HRC 57). Primarily used for nozzles, bearings, valve parts, valve stems, and seats.

• 17-4PH: A precipitation-hardening martensitic stainless steel (~HRC44) combining high strength, hardness and corrosion resistance (comparable to 304/430 in many atmospheric or dilute acid/alkali/sea-water conditions). Not suitable above ~300 °C. Used for offshore platforms, turbine blades, valve components etc.

In practical selection for general use (balancing cost and performance) the usual sequence for austenitic stainless steel is: 304 → 304L → 316 → 316L → 317 → 321 → 347 → 904L. Among these, 317 is used infrequently, 321 is not generally recommended, 347 is for high-temperature corrosion resistance, and 904L is selected only for very special components.