What is the difference between AISI 304 and AISI 316 Stainless Steel?

The two most frequently mentioned codes in food machine quotations: AISI 304 and AISI 316. The difference is not just price; it is the lifespan of your equipment and food safety.

Chemical Difference

Both are austenitic stainless steels. 304: ~%18 chromium + %8 nickel. 316 adds ~%2 molybdenum to this. Molybdenum significantly increases resistance to pitting corrosion in chloride (salt) and acidic environments.

Which product for which?

AISI 304 is sufficient:

Water, milk, dry food, granular products
General body, chassis, covers, conveyor

AISI 316 is recommended/required:

Tomato paste and tomato products (acidic)
Ketchup, pickles, vinegar, lemon-salted recipes
Salty brine products (cheese, olives)
Lines with intense contact with CIP chemicals

Practical Engineering Approach

Making the entire machine from 316 is an unnecessary cost; making it entirely from 304 poses a risk of early corrosion in acidic products. The correct design is hybrid: product contact surfaces 316, structural parts 304. When comparing offers, always ask the question "which surface which material" — the explanation for the price difference between two offers is often this.

Certification and Traceability

Material quality is verified by certification, not by declaration. Having material certificates (EN 10204 3.1) in the delivery file is important for both audits and second-hand value.

Surface treatment is as important as material

Even the same 316 can pose a hygiene risk with poor welding and rough surfaces. It should be asked whether the internal welds on food contact surfaces are ground/polished (Ra value).

Example Decision Table: Material According to Your Product

Drinking milk line → contact surfaces 304 sufficient; if CIP circuit is dense, critical points 316
Tomato paste/tomato line → evaporator, pasteurizer, and filling contact surfaces 316; chassis and covers 304
Ketchup/sauce line → inner surface of mixer tank and filling path 316; body 304
Olive/pickle brine → entire wet path from pump to filling 316 (chloride requires)
Dry food/granular filling → 304 sufficient on all surfaces
Fruit juice → for acidic fruits (pomegranate, sour cherry) contact surface 316 is recommended

This table is for initial guidance; the final decision is made with your product analysis values (pH, salt, temperature).

Material Control Checklist in the Offer

Is the distinction between "product contact surface" and "structural part" written in the offer?
Will the EN 10204 3.1 material certificate be provided upon delivery?
Are internal welds ground/polished (is the target Ra value specified)?
Are seals and plastic contact parts food approved (FDA/EC 1935)?
Is there a declaration of resistance to CIP chemicals?

How Corrosion Starts: Passive Layer and Chloride

What protects stainless steel is the few nanometers thick chromium oxide "passive layer" on its surface. Chloride ions (salt) puncture this layer; invisible pinhole pits (pitting) begin and progress beneath the layer. The advantage of 316 with molybdenum is precisely here: it increases the passive layer's resistance to chloride. Practical results: a 304 surface in brine and acidic products may show "no problems today, leaking two seasons later" behavior; when damage is visible, it is too late. Additionally, improper cleaning also triggers corrosion — leaving chlorine-based bleach on a stainless surface can pit even the highest quality material. Cleaning instructions are as important as material selection.

Maintenance Note: Passivation

The passive layer deteriorates in areas of welding and mechanical damage; passivation with nitric/citric acid restores the layer. It should be required that the welded areas of new equipment are stripped + passivated; this process should also be repeated after major repairs. A single line in the offer stating "passivation included" can make a difference of years in the field.

How to Verify at Delivery?

Do not let the material declaration remain on paper; three practical steps are sufficient at the acceptance stage. First, certificate matching: request from the manufacturer to mark the drawing showing that the casting numbers on EN 10204 3.1 certificates match the critical parts on the equipment. Second, spot verification: measuring a few critical surfaces with a portable XRF (spectral analysis) device takes minutes; for large investments, it can be done by an independent inspection firm. Third, weld area control: visually and photographically record that internal welds are ground/passivated during FAT. These three steps take about one-thousandth of the total project cost in time; if skipped, the bill for the wrong material will come as corrosion years later.

Cleaning Chemicals and Material Compatibility

Even if the material is correctly selected, using the wrong chemical shortens its lifespan. Practical rules: do not leave chlorine-based bleaches on stainless surfaces (the most common homemade cause of pitting); do not exceed manufacturer-approved caustic/acid concentrations in CIP; conduct chloride analysis in facilities using well water — high chloride water quietly leaves traces on 304 surfaces. Request the cleaning instructions in writing from the equipment supplier and include them in operator training; the field equivalent of material science is these three habits.

Frequently Asked Questions

Can 304 and 316 be visually distinguished?

No; they look the same. The distinction is made by certification and spectral analysis.

How much is the price difference?

There is a significant difference based on material; however, its effect on the total machine price remains manageable only when contact surfaces are selected as 316.

What is 316L, and how does it differ from 316?

It is a low carbon version; it increases corrosion resistance in welded areas. It is preferred in the manufacturing of heavily welded tanks and pipes.

Share your product's acid/salt profile; let us evaluate for free which material is right for which equipment. Just write your product in the quotation form.