Durability and life expectancy for stainless steels in external environments

Introduction

The most important corrosion consideration with stainless steels is pitting.
Micro pitting is manifested as surface rust staining and is generally considered unacceptable for a material basically selected for its aesthetic appearance and corrosion resistance.

Analysis and extrapolation of pit depth data gathered from atmospheric exposure test programs can be used as a durability/longevity guide. (This work was done by Corus, (British Steel), Technical)
It must be borne in mind that staining from micro pitting may result in rejection of the steel on aesthetic grounds, long before pitting has perforated it.

Factors affecting durability

The durability/longevity of the stainless steel depends on: –

steel grade

environment

surface finish

 

Chloride ions are the most aggressive environmental hazard when assessing the pitting corrosion risk. Marine sites tend to be the most aggressive and so give the lowest projected durability.
Acid conditions, sometimes found in industrialised atmospheres, are also aggressive.

It is also important to consider the effects of local ‘micro climates’ that may influence how aggressive the environment is.

Estimated pit penetration times

The pitting life predictions shown in the table are based on a linear pit growth rate model.

Location Estimated Time to Penetrate 1mm (Years) by steel type 430, (1.4016) 304, (1.4301) 316, (1.4401) Marine N/A 145 260 Semi-industrial 85 135 525 Rural 250 770 1200

The marine and rural site samples had ‘mill’ finishes and the semi-industrial site results had dull polished finishes.

The 1mm-perforation times for the 304, (1.4301), type are consistent with the projected design lives of most buildings.

It should be noted that the ferritic 430, (1.4016), type is not normally considered for building exterior applications in the UK, but is included in the table for comparison.

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Strictly speaking, very few metals are "stable" in terms of the laws of thermodynamics. True chemical stability is when the atoms are in their lowest energy state. For most metallic elements, various oxides, sulfides, and chlorides are lower energy states than the pure or alloyed metal. This is why corrosion occurs in the first place--the atoms will tend to form compounds that reduce their overall free energy--iron becomes iron oxide, aluminum becomes aluminum oxide, etc.

Most alloys, including stainless steel, are 'metastable', meaning that they will--eventually--move toward a lower energy chemical state (i.e. they will eventually corrode). However the kinetics, or time-dependent processes of corrosion can be slowed down to the point that they appear stable over the normal human lifespan. As others have pointed out, various catalysts and corroding agents can accelerate this process. But even just sitting 'in a museum', degradation will proceed at some vanishingly slow rate.

So the short answer is, "yes, there is a lifespan for all alloys, but sometimes that lifespan is so long that it doesn't matter; and the service environment plays a huge role in how long that takes."

Edit: As an aside, this also means that some of the useful microstructures of steels (pearlite, austenite, martensite, etc.) will also tend to degrade over time since most of these are metastable. Even if the chemical identity of the material doesn't change, the atoms will gradually rearrange themselves over time to reach their most stable configuration. Again, this is usually (but not always) on time scales far beyond what you'll need to plan for. Nevertheless, this is one reason why you should always be aware of the temperature environment of your metals--even if the heated alloy doesn't outright corrode away, a few hundred degrees can drastically reduce the time required for microstructural degradation, which can dangerously weaken hardened steels.