A4-80 vs A2-70 Stainless Steel Bolts: Which One for Marine Applications?

Introduction

If you are specifying bolts for a ship, offshore platform, or any marine structure, you have probably faced this question: should I use A4-80 or A2-70 stainless steel bolts?

Both grades look identical. Both are "stainless steel." But the difference between them can mean the difference between a 20-year service life and a bolt that starts corroding in 2 years.

This article gives you a direct, practical comparison so you can make the right choice for your specific application. No fluff, just the facts you need.

What Do A4-80 and A2-70 Actually Mean?

Before comparing, let us decode the grade markings:

[Source: ISO 3506-1:2020 - Mechanical properties of corrosion-resistant stainless steel fasteners]

The key difference is the steel composition, specifically the molybdenum content:

• A2 (304): 18% Chromium, 8% Nickel, 0% Molybdenum
• A4 (316): 18% Chromium, 10% Nickel, 2-3% Molybdenum

That 2-3% molybdenum is what makes A4-80 resistant to chloride attack from seawater. Without it, A2-70 is vulnerable.

Corrosion Resistance: The Real Difference

This is where the two grades diverge dramatically in marine environments:

[Source: ISO 9223:2012 - Corrosion of metals and alloys; NACE International corrosion data]

Why A2-70 Fails in Seawater

A2-70 lacks molybdenum, which means chloride ions from seawater can break down the passive oxide layer on the bolt surface. Once this layer is compromised, pitting corrosion starts - small, deep holes that can cause sudden bolt failure.

A4-80's molybdenum content stabilizes the passive layer, making it much more resistant to chloride attack. It is not immune, but it lasts significantly longer.

Strength Comparison

Strength matters for structural connections. Here is how they compare:

[Source: ISO 3506-1:2020 Table 3 - Mechanical properties of austenitic steel fasteners]

A4-80 is not only more corrosion resistant, it is also stronger. For structural connections on ships, this makes A4-80 the safer choice.

Price Difference: Is A4-80 Worth the Extra Cost?

Yes, A4-80 costs more than A2-70. But how much more?

While A4-80 costs 20-40% more upfront, the total cost of ownership over 15 years is typically lower because you replace bolts far less often. For critical structural connections, the choice is clear.

Common Mistakes to Avoid

Mistake 1: Using A2-70 Below the Waterline

A2-70 will pit and corrode quickly in submerged seawater. Always use A4-80 or higher grade for below-waterline applications.

Mistake 2: Mixing A2 and A4 Bolts in the Same Connection

Mixing different stainless steel grades can create galvanic corrosion. The less noble metal (A2) will corrode faster. Always use the same grade throughout a connection.

Mistake 3: Assuming "Stainless" Means "Never Rusts"

All stainless steel can corrode in harsh environments. A4-80 is more resistant, but still requires proper selection, installation, and maintenance.

Standards and Certifications

When procuring marine bolts, ensure they comply with relevant standards:

• ISO 3506-1:2020 - Mechanical properties of corrosion-resistant stainless steel fasteners
• ISO 4014 / DIN 931 - Hex head bolt dimensions
• DNV GL - Marine and offshore certification (for structural applications)
• ABS / LR / BV - Other major classification society certifications

[Source: ISO 3506-1:2020; DNV-OS-B101 Metallic Materials]

For structural connections on commercial vessels, classification societies typically require A4-80 minimum for any bolt exposed to marine atmosphere or seawater.

Summary: A4-80 vs A2-70 at a Glance

Bottom line: For marine applications, A4-80 is almost always the right choice. The extra 20-40% upfront cost pays for itself many times over in reduced maintenance and replacement costs.