DIN/ISO A2 Stainless Steel Is A Corrosion Resistant Steel; It Is Also Known As ASTM-304.

A2 / ASTM-304 Is An 18/8 Stainless Steel : This Designates A Metallurgical Content Of 18% Chromium & 8% Nickel.

A Bolt Marked A2-70 Is A 304 Stainless Steel Bolt With A 700 N/mm2 Tensile Strength (See Below Table).
The -70 Representing The Tensile Strength Divided By 10.
-50 & -80 Grades Are Also Available But Are Not All That Common.

A2 / 304 Stainless Steel Is What Is Known As An Austenitic Stainless Steel : It Is (Mostly) Non-Magnetic.

      Though Some Magnetism Can Be Left Residual After Cold-Working.

A2 / 304 Stainless Steel Is Amongst The Most Highly Corrosion-Resistant Materials Available To The Designer And Engineer.

Tensile Strength Denotes The Load At Which The Material Breaks.

0.2% Yield Strength Denotes The Load That Will PERMANENTLY Deform / Stretch The Material By 0.2% Of Its Original Size.
This Is Sometimes Referred To As The Elastic Limit.

A2-70 Can Be Safely Used To Hold Car Body Panels And Bumpers; Wing (Fender) Bolts, Hinge Bolts, Handles, Locks, Light Fittings Etc.

A2-70 Can Be Safely Used On Non-Mechanically Loaded Engine Components Where A Simple Clamping Action Is Required (Subject To Torque Limitations).
i.e: Inlet & Exhaust Manifold Attachment, Ancilliary Attachment, Pipe Clamps, Flange Joining etc.

A2-70 Can Also Be Used For Securing Access Panels, Cover Plates And Accessory Items To The Vehicle.

A2-70 Should Not Be Used In Heavily Loaded Mechanical Or Structural Areas.

• Areas Of Direct Loading On The Suspension.
• Areas Of Direct Loading On The Steering Column And Linkages.
• Active Engine Componets (Con-Rods, Crank Shaft, Clutch & Power Off-Takes)
• Active Gearbox And Power-Train Components.

A General Rule Of Thumb Is That It Is OK To Replace 8.8 And Lower Rated Nuts And Bolts.

Any Nut Or Bolt Stamped With A 10:9 Or 12:9 Should Be Replaced With An Identically Rated (Or Higher) Component AND NOT An A2-70 Component.

The Below Table Gives A Rough Guide To The Maximum Torque Wrench Settings To Be Used On The Most Popular Metric Sizes.

A4 grades of stainless steel are usually reserved for use in highly caustic environments where improved resistance to pitting corrosion is required, i.e Marine & exhausts.
Pitting corrosion occures once the chromium becomes exhausted through self-passivation after caustic attack - the effects as usually local to the exposed areas. The pitting forms as the Iron atoms become exposed to Oxygen and form 'rust'.

The improved resistance comes from the addition of a higher proportion of Chromium, which helps to form a protective oxide layer on the iron - the more chromium you have the longer the protection it offers lasts.

However, whilst some properties are improved, others deteriorate. The extra Chromium makes the metal more succeptable to fatigue and cracking : OK if your rolling serenly around on the high seas : not so OK if you driving a rickety old banger like a VW.!

This extra protection is not necessary for automotive use as the component life of A2-70 grades is generally massive (often lifetime guarantees are given on items made from these materials).
A2-70 components will outlive your vehicle.

Professional Engineers agree that A2 Stainless is the optimum selection for automotive use.

Galvanic Corrosion Takes Place When Two Dissimilar Metals Are Electrically Coupled By An Electrolyt In The Prescence Of Oxygen To Form A Galvanic Cell. The More Reactive Of The Two Metals Will Then Corode (Oxidise) In Preferance To The Less Reactive Metal.

Steel Structures In The Maritime Environment Are Often Protected By Zinc Annodes. The (More Reactive) Zinc Corrodes (Oxidises) And Dissolves In The Prescence Of Salt Water, Thereby Offering Some Anti-Corrosion Protection To The (Less Reactive) Steel. The Servicable Life Of The Steel Is Thus Extended.

The Rate At Which Galvanic Corrosion Can Proceed Is Goverened By Two Key Factors.

• Available Quantity Of Electrolytic Liquid
• The Relative Mass Of The Two Dissimilar Metals Respective To Each Other

Stainless Steel Is Less Reactive Than Ordinary Mild Steel Or Cast Irons ; Therefore The Iron/Steel Will Corrode Galvanically (If The Above Conditions Are Met) Protecting The Stainless Steel.

In Terms Of A Cast Iron Engine Block With Stainless Exhaust Studs The Mass Of The Engine Block Is Massive Compared To The Small Mass Of The Studs. So Any Effects Are Negligable.
Fuel/Oil Misting & Residues Around Engines Will Offer Protection As They Form A Dielectric Barrier To Electron Flow - Preventing Galvanic Cell Formation. Fuel/Oil REsidue Will Also form A Barrier Against Oxygen - The Second Key Ingredient To Galvanic Corrosion.

The Stainless Steel Exhaust Aftermarket Thrives Without Any Reported Galvanic Corrosion Issues Despite The Relative Masses Being Closer Than With A Few Threaded Studs.
Its Also Interesting To Note That The DeLorean Motor Company Also Opted To Make A Whole Car Bodyshell Out Of Stainless Steel.