Shade Structure Corrosion: Is It Preventable?

Corrosivity

What is corrosivity?

I was in the UK recently visiting family and visited Cromer, a seaside resort in Norfolk. They have a few old tractors which they use to drive into the water to drag boats up the slipway. The main one is a classic old Muir-Hill (yeah I haven’t heard of that brand either) and the rust on it is as thick as anything. Anyway, it got me thinking about corrosivity.

According to the Australian Standard, corrosivity is “a measure of the ability of the environment to cause corrosion. Often used interchangeably with the term ‘corrosion rate’.” In layman terms, how fast it rusts (duh). It’s important because Curtin Uni says corrosion costs Australia up to $32 billion annually or $1,500 per person per year. So that’s a lot. And they also add that most of it is preventable, so that’s good.

What does it apply to?

We’re talking from a shade structures and covered areas perspective. The main application is the steelwork, but it also applies to fasteners and fixings, roofing and rainwater products, reinforcing and footing components and ancillary items.

All these products have some corrosion over time, so the goal is to limit this to an acceptable level. This is done with a number of factors in mind, including the corrosivity rate (or risk level), design life of the structure, expected maintenance regime, cost factors and client requirements.

The cost factor is an interesting one, and any astute buyer would realise the benefit of investing a little more upfront for a longer product lifespan. There are two factors to “lifespan” – firstly the Life to First Maintenance, and secondly the Design Life. Both should be considered in any comparison of products.

What are the risk levels?

Corrosion is primarily caused by two main factors – salt or industrial pollutants. I’m not going to get too much into the need for oxygen and electrolytes, nor into micro- and macro-climates. (Go read the standard nerd.) But in simple terms, the closer you are to the ocean, especially rough oceans, or to polluting factories (carbon tax anyone?), the higher the risk.

There are a few other common things we come across:

• Abrasions (someone using a whipper snipper around the bottom of the posts, or kids scratching the paint off)
• Structures around swimming pools (all that chlorinated water)
• Poor structure design (nice little pockets that catch water)
• Poor coating application (1^st day apprentice)
• Poor product specification (i.e. just read this guide and you’ll be OK)
• Dissimilar metals (stainless steel touching mild steel – a big no-no)

The standard has some complicated maps and guidelines for defining the Corrosivity Category, but this picture shows it better. OK, not so technical, but easier to understand. In essence, nearly no C1 exists in Aus. C2 is common inland (for Sydney, sort of west of Parramatta, and much closer to the coast in Melbourne), C3 for coastal, and C4-C5 for very near the coast.

Now Bluescope has decided to word it differently (to be difficult?), but once again it is based on distance from the pollution source. I don’t know if they’ve had a good look at the corrosion rates in Newcastle, but I think they have a mill there, so I presume someone has.

What products can be used?

Right, so down to business. What can I actually use that will last? The below table is quite a generic guide but it should provide some pointers. Best thing to do is give us a call if unsure.

What about maintenance?

Maintenance is probably the largest overlooked item in the longevity of a structure. In an area with high levels of airborne salt, simply washing the structure with tap water every 6 months can majorly reduce the risk of corrosion. Now there’s some technical stuff in here and Colorbond has some lengthy guides, so give us a ring for more details. The one takeaway: the small cost of regular maintenance is much cheaper than the large cost or re-coating or replacing.

What standards and references apply?

This is a non-exhaustive list of standards that apply to the topic:

• AS2312 (Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings)
• AS 4312–2008 (Atmospheric corrosivity zones in Australia)
• AS 3566
• ISO 9223 (Corrosion of metals and alloys – Corrosivity of atmospheres – Classification)

And these are some sites that provide further reading:

• Galvanizers Association of Australia
• Dulux, Interpon and other protective coating suppliers
• Bluescope

Sources & acknowledgements

• Dr Reza Javaherdashti – Research Fellow at Curtin University School of Civil and Mechanical Engineering
• Bluescope – steel grades