What is the difference between 304 and 316 stainless steel?

316 contains 2–3% molybdenum, which raises its resistance to pitting and crevice corrosion in chloride environments; 304 is more economical for drinking water and general use.

For extensive welding, pharmaceuticals, WFI and hygienic systems; carbon below 0.03% reduces the risk of sensitization.

What is Ra and why does it matter?

Ra is the average surface roughness; a lower Ra means easier cleaning and less biofilm. Pharmaceutical work often requires Ra < 0.8 µm, and critical uses < 0.4 µm.

What does passivation do?

After welding it removes free iron and heat tint and rebuilds the passive chromium-oxide layer, so long-term corrosion resistance is kept.

Stainless steel tank fabrication

Stainless steel tanks for the water industry · grades, corrosion, surface finish and hygiene standards

A stainless steel tank is for fluids that must not react with the plate or be contaminated: drinking water, RO water, milk and food, pharmaceuticals and many chemicals. Stainless steel has a protective chromium-oxide layer that re-forms if it gets scratched, so a stainless tank needs no internal lining.

Request a quote Specifications

Steel grade

304 · 304L · 316 · 316L

Internal finish

2B to electropolish · controlled Ra

Welding

GTAW / orbital · purging

Surface treatment

Pickling + passivation

Grades 304 · 304L · 316 · 316L

Self-healing passive layer with at least 10.5% chromium

Controlled surface finish (Ra) up to electropolish

Orbital GTAW welding with back purging

Pickling and passivation after welding

Compliant with 3-A · FDA · ASME BPE · NSF/ANSI 61

Stainless steel tank fabrication

Stainless steel tanks for water, food, pharmaceuticals and chemicals

The fluid decides the grade. 304 is enough for water and most foods. Where there are chlorides or acids, 316 or 316L is needed. In food and pharmaceutical tanks, the quality of the welds and the internal surface finish matter as much as the grade itself, because bacteria settle in rough surfaces and uneven welds.

Behineh Ab builds stainless steel tanks from 500 to 100,000 litres, single-wall, double-wall and with agitators. A stainless tank weighs about the same as a carbon steel one, but stainless plate costs several times more per kilo. We price tank fabrication on that weight. Below we explain grades, corrosion, surface finish, welding and passivation.

Why stainless steel?

The passive layer

Stainless steel contains at least 10.5% chromium. Chromium reacts with oxygen to form a very thin, dense, self-healing chromium-oxide layer that protects the base metal from corrosion — the secret of being 'stainless'.

2. Common stainless steel grades

2-1. Grades 304 and 304L

The most common grade in the world ('18-8': about 18% Cr and 8% Ni). Suited to drinking water, hot and cold water and general food use. Low-carbon 304L is for heavy welding and reduces sensitization and intergranular corrosion.

2-2 and 2-3. Grades 316 and 316L

316, with 2–3% molybdenum, has higher resistance to pitting and crevice corrosion in chlorides (sea water, brackish water, salt) — the standard for pharmaceuticals and corrosive environments. 316L, with C < 0.03%, keeps its resistance in the heat-affected zone and is the gold standard for WFI, biotech and sanitary service.

Grade comparison

Grade	Key composition	Corrosion resistance	Typical use
304	18% Cr · 8% Ni	Good	Drinking water, general tanks, food
304L	Low-carbon 304	Good (better welds)	Heavily welded structures
316	+ 2–3% Mo	Excellent	Chlorides, sea water, pharmaceuticals
316L	Low-carbon 316	Excellent (best welds)	WFI, biotech, sanitary

Selection tip

Chlorides, sea water or strict pharmaceutical requirements → 316/316L. Ordinary drinking water → 304 is more economical. Extensive welding → the L version.

3. Reasons to choose stainless steel

3-1. Corrosion resistance

The passive layer rebuilds itself in the presence of oxygen even after a scratch. Decades of life without an internal lining. Uniform corrosion is rare; pitting and crevice corrosion are handled with 316 and a proper finish; intergranular corrosion with L grades and passivation.

3-2 and 3-3. Hygiene and CIP/SIP

The non-porous surface limits bacterial attachment and biofilm and gives no taste or odour to the water. In food and pharmaceutical plants it is cleaned and sterilised by CIP/SIP and withstands steam temperatures.

3-4. Other advantages

High durability, a wide temperature range, mechanical strength, 100% recyclability and a bright finish suited to industrial and display settings.

4. Surface finish and the Ra parameter

The lower the Ra, the smoother the surface, the easier the cleaning and the fewer places for bacteria to collect:

2B mill finish

Ra ≈ 0.5–1.5 µm

Mechanical #180

Ra ≈ 0.8 µm

Mechanical #320

Ra ≈ 0.4 µm

Electropolish

Ra < 0.4 µm

Why does a smooth surface matter?

A rough surface is a landscape of microscopic peaks and valleys that traps dirt and bacteria. Pharmaceutical work usually requires Ra < 0.8 µm (32 µin), and critical uses Ra < 0.4 µm. Electropolishing leaves a chromium-rich surface and is the usual standard for WFI and biotech.

4-2. Electropolishing

Process and result

An electrochemical process selectively removes a thin surface layer; microscopic peaks are levelled and the result is a bright, chromium-rich surface with better corrosion resistance and hygiene — the usual standard for WFI and pharmaceuticals.

5. Orbital welding and passivation

5-1. Orbital welding

Mechanised GTAW/TIG in which the weld head rotates around the joint under control. Consistent quality, full penetration, minimal heat tint, and digital recording of parameters for validation. In pharmaceutical and ultrapure water service it is effectively required. Argon back purging prevents oxidation of the root.

5-2. Passivation

After welding and machining, free iron particles and heat tint reduce corrosion resistance. Passivation with nitric or citric acid removes free iron and rebuilds the passive chromium layer. It usually follows pickling, which removes heavy heat tint.

Final surface-treatment steps

1) Degreasing 2) Pickling 3) Rinsing and neutralising 4) Passivation 5) Final rinse, drying and inspection. Heat tint after welding is a sign of chromium depletion at the surface and must be removed completely.

6. Hygiene and quality standards

6-1 to 6-3. Key references

3-A: sanitary design for food, dairy and beverage. FDA: materials in contact with food and drugs (304/316 approved; gaskets and seals must comply too). ASME BPE: biopharmaceutical equipment. EHEDG: European hygienic design. ASME VIII: pressure vessels. NSF/ANSI 61: drinking-water system components. EN 10088: material specifications.

Standards table

Standard	Scope
3-A	Food, dairy and beverage equipment
FDA	Materials in contact with food and drugs
ASME BPE	Biopharmaceutical equipment
NSF/ANSI 61	Drinking-water system components
ASME Sec VIII	Pressure vessels

7. Uses in the water industry

7-1 to 7-5

Drinking-water storage in buildings and networks. RO and ultrapure/distilled water systems in 316L with electropolish. Pressurised booster and expansion tanks. Food, pharmaceuticals and biotech with CIP/SIP. Storage and dosing of water-treatment chemicals in a suitable grade.

Applications at a glance

Use	Recommended grade	Key point
Drinking water	304 / 316	Hygienic surface, no taste transfer
Ultrapure water and RO	316L	Electropolish, low Ra
Booster and expansion	304 / 316	Pressure rating, ASME VIII
Pharma and biotech	316L	CIP/SIP, BPE, orbital welding
Chemicals	316 and above	Chosen by the chemical

8. Summary

By combining corrosion resistance, hygiene, durability and strength, stainless steel tanks are a reliable choice for the water and hygienic industries. Grade selection, Ra quality, orbital welding and passivation decide their service life; following 3-A, FDA and ASME BPE keeps the product safe.

Behineh Ab Engineering offers specialist advice on grade selection and designs and builds stainless steel tanks for drinking water, industry, food and pharmaceuticals.