Corrosion costs South Africa dearly. The Corrosion Institute of Southern Africa puts the direct cost at about 4% of GDP, with indirect costs at least three times higher. Yet many asset owners still specify the same anti corrosion coating for structural steel they used decades ago.
Often the problem is not the steel, but the coating. If a coating cracks with thermal movement, breaks down in UV or needs blasting before every recoat, you pay for a maintenance cycle you could avoid. This article explains why silicone changes that, and why it suits African conditions so well.
Why Structural Steel Corrodes Faster in Southern Africa
Our conditions are hard on steel. Along the coast, from Durban to Dar es Salaam, salt air settles on every surface. Industrial areas add sulphur dioxide and other airborne chemicals. Inland, steel heats and cools sharply between cold mornings and hot afternoons.
UV levels in Southern Africa are among the highest in the world. As a result, coating binders that last well in cooler climates break down faster here. Chalking, cracking and loss of adhesion follow. Once a coating fails at one spot, moisture gets in and rust creeps under the film, long before anyone sees it.
The outcome is a maintenance cycle that never ends: strip, blast, recoat, repeat.
The Problem With Conventional Anti Corrosion Coating on Steel
Epoxy dominates the South African market. It offers strong adhesion and chemical resistance, and applicators know it well. However, it has one big limit outdoors: it is rigid.
Steel moves. It expands in heat and contracts in cold. A rigid coating cannot follow that movement for ever. Fine cracks form, moisture enters, and corrosion starts underneath. By the time rust shows, the damage is already serious.
Epoxy systems also need abrasive blasting for proper preparation. On a structure in service, blasting is disruptive, costly and often impractical. It creates waste, needs containment and takes structures out of use for long periods. In addition, zinc rich primers need precise thickness and curing time before the next coat.
For new builds in a controlled shop, epoxy is still a capable choice. For maintenance and retrofit on structures in service, however, it is often the wrong tool.
What Makes a Silicone Anti Corrosion Coating for Steel Different
Silicone behaves differently from epoxy. Where epoxy cures rigid, silicone stays flexible. That flexibility is exactly what gives it a long life outdoors.
Si-COAT 579CM from CSL Silicones is a single component, moisture cure RTV silicone coating built for corrosion maintenance and retrofit work. Its tested properties include:
- 180% elongation at break, so the coating flexes with the steel instead of cracking
- No degradation after 5,000 hours of accelerated UV weathering to ASTM G154, with a colour change below 1 delta E
- A pass in 3,000 hours of ASTM B117 salt fog
- A pass in the 36 month Florida Fence Test with scribe: no rust creep, bleed through, pinholes or loss of adhesion
- An unlimited recoat window, so you can overcoat it at any time after cleaning
The Florida Fence result stands out. In that test, the lab rusted the panels, cleaned them with hand tools only, and applied Si-COAT 579 with no primer. By contrast, three coat zinc, epoxy and polyurethane systems on blasted steel showed heavy rust creep.
The Retrofit Advantage: No Abrasive Blasting
This is where Si-COAT 579CM changes the economics of maintaining structural steel.
Conventional systems set abrasive blasting as the minimum. On a retrofit job, that means scaffolding, containment, blasting equipment, waste disposal and downtime. On bridges, tanks and structures in service, surface preparation is often a large share of the total cost.
Si-COAT 579CM needs SSPC SP2 hand tool cleaning, SSPC SP3 power tool cleaning or SSPC SP12 water jetting to WJ-4. Crews remove loose rust, peeling paint and contamination, and the surface must be clean and dry. No abrasive blasting or profiling is needed, so there is no need for a blast crew on site.
It is also self priming. First, brush a stripe coat of about 0.13 mm dry (5 mil) onto edges, welds, bolts and pits. Then apply one full coat of about 0.2 to 0.5 mm once dry (8 to 20 mil). At standard conditions, it is tack free in 60 minutes, dry in 4 to 6 hours and reaches full properties in 7 days.
Where steel stands on a concrete plinth, we carry one continuous coat from the concrete onto the steel, so no joint is left for water. First, though, remove laitance and any curing compound from the concrete.
For the harshest coastal and industrial sites, CSL also pairs it with CSL 979 primer. That system passed ISO 12944-6:2018 testing for C5 High. The C5 result applies to the system only, not to Si-COAT 579CM on its own.
Where Si-COAT 579CM Is Specified
Si-COAT 579CM is for above ground atmospheric exposure in retrofit and maintenance work. Typical uses include:
- Structural steel frameworks and support structures
- Road and rail bridges
- Tank exteriors
- Machinery and industrial equipment
- Metal roofs and cladding
- Steel with existing heavy corrosion
Crews apply it by airless spray on large areas, or by brush and roller on smaller sections and spot repairs. The surface must be between 5°C and 60°C and at least 3°C above the dew point. On cold highveld mornings, therefore, wait for the steel to warm up.
Where you also need a silicone roof coating or high voltage insulator protection, we supply the full CSL Silicones range.
FAQ About Anti Corrosion Coating for Structural Steel
What is the best anti corrosion coating for structural steel?
It depends on the job. For new construction in a controlled shop, zinc rich epoxy systems work well. For maintenance and retrofit on structures in service, however, a flexible silicone such as Si-COAT 579CM is often the better choice. It flexes with the steel, resists UV and goes on without abrasive blasting. Where C5 performance is required, specify the Si-COAT CM System with CSL 979 primer.
Do you need to sandblast before applying anti corrosion coating to steel?
Not with Si-COAT 579CM. Most conventional coatings need blasting for good adhesion. By contrast, Si-COAT 579CM primes itself and needs only hand tool, power tool or water jet cleaning. You still remove loose rust and peeling paint, just not by blasting. As a result, preparation is quicker and disruption is lower.
How long does anti corrosion coating last on structural steel?
It depends on preparation, thickness and exposure. Our benchmark for Si-COAT 579CM is about 20 years, based on a CSL Florida power plant pipe coated in 2003, which still protects its steel. It has also passed the 36 month Florida Fence Test with no rust creep, and 5,000 hours of UV weathering with no degradation.
Can you apply anti corrosion coating over existing paint or rust?
Yes, within limits. Old coating that stays must pass the ASTM D3359 tape test at 4A or 4B. First, remove any paint that is peeling, flaking or blistering, plus all loose rust. Tightly adhered rust that hand or power tools cannot lift can stay. In CSL testing over an aged coating, the Si-COAT layer also bridged cracks in the old paint without damage.
What is the difference between silicone and epoxy coatings?
Epoxy cures rigid and gives strong early adhesion. Outdoors, however, it cracks with thermal movement and its surface chalks in UV, so it needs a topcoat. Silicone cures flexible and stays that way. Si-COAT 579CM reaches 180% elongation at break, so it moves with the steel. It is also UV stable without a topcoat. For more on the product range, visit CSL Silicones.
What causes corrosion on steel structures in Southern Africa?
Corrosion is an electrochemical reaction between steel, oxygen and moisture. In Southern Africa, several things speed it up. Coastal air carries chlorides that attack steel through weak spots in a coating. Industrial areas add sulphur dioxide. Strong UV breaks down coating binders, and heat cycles crack rigid films. The impact goes beyond structures, too. A US study found that more than half of unplanned power outages relate to corrosion.
Protect Your Steel Assets With Si-COAT 579CM
Whether you run a bridge maintenance programme, protect industrial equipment or specify a retrofit coating for structural steel, our team can advise on the right approach. See also our articles on bridge corrosion protection and mining corrosion protection, or read the Si-COAT 579CM FAQ.
Technical Solution Supplies distributes CSL Silicones products across Southern Africa, including the DRC and the Seychelles. Contact us to discuss your project.
Proud exclusive distributor of CSL Silicones in Southern Africa.