RST vai HST – kumpi valita? Ruostumattoman ja haponkestävän teräksen vertailu

The quality of stainless steel affects the material's lifespan, price, and the project's risk level. The two most common grades — RST (AISI 304) and HST (AISI 316) — look identical, but their chemical composition and environmental resistance differ. The wrong choice can lead either to an unnecessarily high price or, conversely, to corrosion marks appearing after a few years.

The aim of this article is to help the project manager, installation company, or architect make an informed choice before sending a request for quotation. We cover chemical composition, mechanical properties, corrosion resistance, price differences, and practical rules for material selection.

What does RST mean?

In Finland, RST generally refers to stainless steel — specifically AISI 304 grade. This is the most widely used austenitic stainless steel. Its composition is approximately 18% chromium and 8% nickel, which is also where the name "18/8 steel" comes from. The carbon content is low (less than 0.08%), which prevents chromium from binding with carbon and maintains corrosion resistance even after welding.

According to the European EN standard, AISI 304 corresponds to grade 1.4301. The low-carbon version is AISI 304L or 1.4307 — this is recommended for welded, thicker structures to avoid sensitization (precipitation of chromium carbides at grain boundaries).

RST properties in brief:

• Corrosion resistance against fresh water, air, and many food products

• Good weldability and formability

• Non-magnetic in its basic state (cold working can make it slightly magnetic)

• Operating temperature temporarily up to approximately 870 °C

• EN 10204 3.1 material certificate typically included with each batch

Applications: kitchen appliances, indoor handrails, architectural details, food industry equipment (if no contact with salt water), sanitary pipes, interior elements.

What does HST mean?

HST, or acid-resistant steel, corresponds to AISI 316 (EN 1.4401). It is also austenitic but additionally contains 2–3% molybdenum. The role of molybdenum is the most important technical detail of the entire article: it prevents pitting and crevice corrosion in environments containing chlorides, salt, or acids.

The low-carbon version is AISI 316L (EN 1.4404) — the same logic as 304L, recommended for welded structures.

HST properties compared to RST:

• Withstands salty water and chlorides

• Better performance in acids (sulfuric acid, formic acid, acetic acid in moderate concentrations)

• Slightly higher strength at high temperatures

• Non-magnetic in its basic state

Applications: coastal locations, swimming pools and saunas, shipbuilding, chemical industry, pharmaceutical equipment, outdoor stair railings in coastal areas, aquaculture and food industry equipment exposed to salt water.

Chemical composition side by side

The table below shows the key compositional differences between the two grades according to the EN standard.

The key difference is the 2–2.5% molybdenum addition and slightly higher nickel content in HST. It is precisely these two components that explain the price difference — both are significantly more expensive on the raw materials market than chromium or iron.

Mechanical properties

In terms of strength, the grades are close to each other but not identical.

In practice, this means that in a construction project, there is no mechanically significant difference between the two grades. The choice is made based on environment and price, not strength.

Corrosion resistance — where the real difference lies

Stainless steel is not "corrosion proof" in an absolute sense. Chromium forms a thin passive layer on the surface that prevents oxidation. However, this layer can break down locally if the environment contains chlorides, acid, or high temperatures combined with moisture.

One common metric for comparing corrosion resistance is PREN (Pitting Resistance Equivalent Number):

PREN = %Cr + 3.3 × %Mo + 16 × %N

Typical values:

• RST / AISI 304: PREN ≈ 18–20

• HST / AISI 316: PREN ≈ 24–26

The higher the PREN, the better the material resists pitting in salty environments. The difference between 18 and 25 seems small, but in practice, it means that RST in a coastal environment can start to develop brown rust spots within just a couple of years, whereas HST maintains its surface for ten years or longer.

Where RST faces issues

• In coastal air (salt aerosol)

• In swimming pool and sauna environments (chlorine as a disinfectant)

• In close proximity to road salt (parts near roads in winter)

• In chemical processing (HCl, H₂SO₄ even in low concentrations)

• In the food industry where there is direct contact with saltwater

Where HST is sufficient

• Baltic Sea and Atlantic coastal areas

• Outdoor and indoor swimming pools

• Food and beverage industry

• Pharmaceutical and chemical industry at moderate concentrations

For extremely aggressive concentrated acids, even HST is no longer sufficient — in these cases, duplex or super-austenitic grades are used (for example, 904L, 2205), but this falls outside the scope of this article.

Price and availability

The price difference between the two grades varies depending on the market situation, but on average, HST is 30–50% more expensive as a raw material than RST. The difference is mainly due to the price of molybdenum and the higher nickel content.

Practical observations on price:

• Standard manufactured sheets and standard-sized pipes are stock items in RST; HST is less frequently in stock and may require a special order.

• The price difference for certified batches (EN 10204 3.1) is more stable.

• Special custom lengths, rolled profiles, or polished surfaces add the same percentage premium to both grades.

If the project's lifespan does not require the properties of HST, choosing the more expensive grade is a cost that does not yield a return on investment.

Practical rule for material selection

Most decisions boil down to a few simple questions.

Use RST (AISI 304) when:

• The project is indoors (kitchen, residence, office, warehouse)

• There is no direct chloride source in the environment

• It is an interior decorative or architectural detail

• A lifespan of 10–20 years is sufficient

Use HST (AISI 316) when:

• The location is outdoors on the coast, within a 10 km radius of the sea

• It is a swimming pool, sauna, or underwater piece of equipment

• There are chlorides (chlorine, bromine, iodine) or acids in the environment

• In the food industry where there is direct contact with saltwater

• The lifespan is 25+ years and maintenance access is limited

In ambiguous cases, most project managers choose HST — more expensive at the time, but without the risk in the long term. This is a justified choice if rebuilding the project would cost substantially more than the price difference of the material.

Common designations and standards

Stainless steel is designated by several systems, which can initially cause confusion.

When ordering material in Europe, it is clearest to refer to the EN number (1.4301 or 1.4401). AISI numbers are common in product catalogs, but the material certificate usually contains the EN code.

How to distinguish RST and HST in the field?

Visually, the two grades cannot be distinguished — the surface is similar, the color is the same, the weight is almost identical. Practical field methods:

• Material certificate (3.1 EN 10204) — the most reliable source

• Molybdenum test (chemical spot testing) — a solution is applied to the surface, and its color change indicates the presence or absence of molybdenum.

• XRF analyzer — a portable X-ray fluorescence device that gives the composition in seconds. A standard tool in quality control.

• The magnet test alone does not work — both are austenitic and non-magnetic in their basic state; with cold working, both can become slightly magnetic. A magnet does not distinguish between RST and HST.

For larger orders, we always recommend requesting a material certificate — it is included in the price and protects the project from subsequent disputes.

When both are suitable — which one to choose?

There are situations where both grades are suitable, and the choice is made based on other criteria. For example, the body of a kitchen appliance can be either RST or HST. The exterior surface of food industry equipment, which comes into contact with detergents but not saltwater, is technically adequate with RST, but HST can extend maintenance intervals.

In such borderline cases, the basis for selection should consider:

• Maintenance access — can the equipment be easily replaced?

• Visual quality requirements — are rust spots acceptable or not?

• Warranty and insurance terms — some insurance companies require HST in coastal locations

• Long-term total cost — 25 years of HST can be cheaper than 15 years of RST + replacement

Frequently Asked Questions

What is the official difference between RST and HST? RST is a general term for stainless steel (in practice, AISI 304). HST stands for acid-resistant steel, i.e., AISI 316, which additionally contains 2–3% molybdenum. Molybdenum provides better resistance against chlorides and acids.

What is the difference between AISI 304 and 1.4301? Same material, different standards. AISI is an American designation, while EN is a European standard. 1.4301 and AISI 304 refer to the same grade.

Is AISI 316 magnetic? Not in its basic state. After cold working (e.g., extrusion or bending), the surface can become slightly magnetic. A magnet test does not distinguish between 304 and 316.

Is AISI 316 suitable for drinking water systems? Yes, and both grades (304 and 316) are food-grade. HST is preferred if the water system contains chlorine or chlorine-based disinfectants.

How long is the lifespan of HST in a coastal environment? With proper installation and light maintenance, 25–40 years. RST in the same environment typically lasts 5–10 years before visible surface damage occurs.

When to use 304L or 316L (low-carbon versions)? When the structure requires welding between larger profiles (e.g., joining thick-walled pipes). Low carbon prevents sensitization near the weld seam.

Does Kominox supply both grades? Yes, AISI 304 is a stock item at Kominox, and AISI 316 is available both in standard sizes and as a special order. Ask for a quote for the exact size and quantity.

Summary and next steps

The difference between RST and HST is based on the presence of molybdenum. For indoor applications and locations further from the sea, RST fulfills its purpose. In the proximity of salt water, chlorides, and more aggressive chemistry, HST is the choice that preserves the surface and reduces subsequent costs.

At Kominox, both grades are in stock. The material certificate EN 10204 3.1 is provided with the batch so that the project documentation meets building regulation requirements.

Request a quote for your specific project — send us the dimensions, quantity, and application environment, and we will respond within one business day.

Related Content

• RST pipe sizes and wall thicknesses — complete table

• Handrail installation — Finnish building regulations

• Welded vs seamless RST pipe — when to choose which

• Service page: RST pipes

• Service page: Stainless steel supplier in Finland