Stainless Steel vs Galvanised Steel

Introduction

Stainless Steel vs Galvanized Steel, it’s crucial to consider the environment, required durability, and maintenance needs. Stainless steel offers unmatched corrosion resistance, strength, and visual appeal, making it suitable for demanding applications in harsh environments. Galvanized steel, on the other hand, offers cost-effective corrosion protection for less aggressive settings.

1. Composition and Manufacturing Process

Stainless Steel

Stainless steel is an alloy composed mainly of iron, chromium (at least 10.5%), and sometimes nickel and molybdenum. Chromium forms a protective oxide layer on the surface, giving it excellent corrosion resistance. Different grades, like 304 and 316, vary in alloying elements, providing options for various environments, including extreme temperatures and high salinity.

Galvanized Steel

Galvanized steel is carbon steel coated with a layer of zinc. The zinc layer protects the steel underneath as a barrier against corrosion. The most common galvanizing method is hot-dip galvanizing, where the steel is submerged in molten zinc. Another method is electro-galvanizing, where zinc is applied using an electric current. Both processes enhance corrosion resistance, though they are generally less durable in harsh environments than stainless steel.

2. Corrosion Resistance

Stainless Steel

Stainless steel’s corrosion resistance is inherent due to its alloy composition, which forms a passive chromium oxide layer. Grade 316 stainless steel, which includes molybdenum, provides excellent resistance to corrosion from chlorides, acids, and other aggressive chemicals. It’s a preferred choice in marine, chemical processing, and oil and gas industries, where exposure to corrosive agents is daily.

Galvanized Steel

The zinc layer on galvanized steel provides sacrificial protection; the zinc will corrode before the underlying steel, offering some corrosion resistance. However, this protection is limited, as the zinc layer can degrade over time. While galvanized steel performs adequately in mild environments and general construction, it doesn’t withstand harsh chemicals or saltwater exposure as effectively as stainless steel.

3. Mechanical Properties and Strength

Stainless Steel

Stainless steel is generally more robust than galvanized steel, with higher tensile strength and durability. This makes it ideal for applications that require resilience and reliability under pressure. Stainless steel also offers excellent resistance to impact and wear, which benefits infrastructure and heavy-duty industrial applications.

Galvanized Steel

While galvanized steel’s strength primarily comes from the carbon steel core, it is generally less robust than stainless steel. The added zinc layer doesn’t significantly contribute to its strength. Galvanized steel is suitable for medium-duty applications where corrosion resistance is necessary but not in extreme or high-stress environments.

4. Appearance and Aesthetics

Stainless Steel

Stainless steel has a sleek, shiny appearance and is often desirable in architectural applications and visible installations. Its aesthetic appeal and durability make it a preferred choice for high-visibility structures and equipment.

Galvanized Steel

The zinc layer gives galvanized steel a dull, matte-gray finish less visually appealing than stainless steel. Over time, exposure to weather may lead to a whitish patina on the surface, which can reduce aesthetic appeal, though it doesn’t impact performance.

5. Cost Considerations

Stainless Steel

Stainless steel is typically more expensive due to its alloying elements, chromium and nickel, and complex manufacturing processes. However, its longer lifespan and minimal maintenance can offset the initial cost, especially in demanding environments.

Galvanized Steel

Galvanized steel is more economical than stainless steel, especially for short- to medium-term applications. It’s a cost-effective choice for projects with a limited budget and moderate corrosion resistance needs.

6. Typical Applications

Stainless Steel Applications

Oil and Gas: Used in pipelines, storage tanks, and offshore platforms due to its high corrosion resistance and strength.
Chemical Processing: Excellent for environments where exposure to acidic or caustic chemicals is every day.
Marine Engineering: Stainless steel’s resistance to saltwater makes it suitable for marine applications like docks, vessels, and equipment.
Infrastructure: Ideal for bridges, railings, and architectural structures where durability and aesthetics are essential.

Galvanized Steel Applications

General Construction: Commonly used in building frames, fences, and roofing supports.
Agricultural Equipment: Provides a balance of corrosion resistance and cost-effectiveness for equipment exposed to soil and moisture.
Water Treatment Facilities: Suitable for non-critical water infrastructure, such as piping and storage tanks in low-corrosion environments.
Outdoor Structures: Commonly used in road barriers, guardrails, and poles, where exposure to mild weather conditions is expected.

7. Maintenance and Longevity

Stainless Steel

Stainless steel requires minimal maintenance due to its inherent corrosion resistance. However, in harsh environments, periodic cleaning is recommended to remove salt, chemicals, or deposits that could compromise the protective oxide layer over time.

Galvanized Steel

Galvanized steel requires regular inspection and maintenance to keep the zinc layer intact. If the zinc layer is scratched or degraded, re-galvanizing or additional coatings may be necessary to prevent corrosion. This is particularly important in marine or industrial applications, where the zinc layer is at risk of degrading faster.

8. Example: Stainless Steel vs Galvanized Steel

PROPERTY	STAINLESS STEEL (316)	GALVANISED STEEL	COMPARISON
Mechanism of protection	A protective oxide layer that self-repairs in the presence of oxygen, granting long-term corrosion resistance.	A protective zinc coating is applied to the steel during manufacturing. When damaged, surrounding zinc cathodically protects the exposed steel.	The stainless steel protective layer is more durable and can ‘heal’ itself. Stainless steel protection does not diminish with material loss or thickness reduction.
Appearance	Many finishes are available, from very bright electropolished to abrasive linished. Appealing high-quality look and feel.	Spangles possible. The surface is not bright and gradually changes to a dull grey with age.	Aesthetic design choice.
Surface feel	It is very smooth and can be slippery.	It has a coarser feel, which becomes more apparent with age.	Aesthetic design choice.
Green credentials	It may be reused in new structures. After the lifetime of the structure, it is valuable as scrap, and because of its collection value, it has a high recycling rate.	Carbon steel is generally scrapped at end-of-life and is less valuable.	Stainless steel is extensively recycled both within manufacturing and at end-of-life. All new stainless steel contains a substantial proportion of recycled steel.
Heavy metal runoff	Negligible levels.	Significant zinc runoff off, especially early in life.	Some European highways have been changed to stainless steel railings to avoid environmental zinc contamination.
Lifetime	Indefinite, provided surface is maintained.	Slow general corrosion until the zinc dissolves. Red rust will appear as the zinc/iron layer corrodes, and finally, the substrate steel. Repair is required before ~2% of the surface has red spots.	Clear life-cycle cost benefit for stainless steel if extended life intended. The economic break-even point can be as short as six years, depending on the environment and other factors.
Fire resistance	Excellent for austenitic stainless steels with reasonable strength and deflection during fires.	Zinc melts and runs, which may cause the failure of adjacent stainless steel in a chemical plant. The carbon steel substrate loses strength and suffers deflection.	Stainless steel offers better fire resistance and avoids the risk of molten zinc if galvanised is used.
Welding on site	This is a routine for austenitic stainless steels, with care about thermal expansion. Welds can be blended into the surrounding metal surface. Post-weld clean-up and passivation are essential.	Carbon steel is readily self-weldable, but zinc must be removed because of fumes. If galvanised and stainless steel are welded together, any zinc residue will embrittle the stainless steel. Zinc-rich paint is less durable than galvanizing. In severe marine environments, crusty rust can appear in three to five years, and steel attacks occur at four years/mm afterward.	Short-term durability is similar, but a zinc-rich coating at joins requires upkeep. In severe conditions, galvanized steel will get rough rust—even holes—and possible hand injury, especially from the unseen seaward side.
Contact with damp, porous material (e.g., wooden wedges) in a salty environment.	It will likely cause rust stains and crevice attack but not structural failure.	Similar to storage stains, it leads to rapid zinc loss and longer-term due to perforation.	It is not desirable for either, but it can cause failure at the base of galvanized poles in the long term.
Maintenance	It can suffer from tea staining and micro-pitting if not adequately maintained.	It can suffer general zinc loss and subsequent corrosion of the steel substrate if not adequately maintained.	Rain in open areas or washing in sheltered regions is required for both.