Hardly out of the ovens, iron and steel oxidize. Once exposed to the vagaries of the weather, metal gates, fences and garden furniture also suffer the effects of corrosion.
To overcome this inevitable rust, we present in detail the reasons for the appearance of rust as well as the anticorrosion treatments.
How Rust Forms
Corrosion, more commonly known as “rust”, is the result of an electrochemical reaction in which four essential factors come into play.
- The anodes (positive charges)
- Cathodes (negative charges)
- Humidity, which promotes electrolysis between anodes and cathodes
- The ambient air which supplies the oxygen necessary for the formation of oxides and therefore corrosion
Yes that’s right. If a metal is completely immersed in a water and no air or oxygen is introduced, it won’t rust. But since there is oxygen in water (else fish wont survive) metals still rust in water.
If something has this rusty look, you probably want to erase the corrosion that is eating away at ironwork, garage doors, furniture, beams. If you are having a serious problem, you can use a few anti-corrosion products which can delay or even stop the corrosive effect.
Why Not Simply Paint It?
Paint Is good. But whatever the qualities of the pigments used in the composition of these anti-corrosion paints, the results are generally not up to par.
The reason? These paints actually only cover easily accessible surfaces. They do not penetrate deep into the sound metal because of their viscosity. They can therefore only partially play their role.
Additionally, applied to porous surfaces, a coat of anti-rust paint traps air and moisture between it and healthy metal. The result corrosion develops rapidly in this location. Rust will appear after blistering and cracking the paint film that covered it. This is called “transplanting”.
To avoid this risk of transplanting as much as possible, one operation is necessary, carefully prepare the surface to be treated against corrosion via mechanical processes such as sandblasting, shot blasting, pitting, scraping, metal brushing, etc.
These operations will not guarantee you 100% to overcome the corrosive effect on your metals. These will take a tremendous amount of time and end up costing money.
Treatments
Compared with other metals, zinc comes first with 150,000,000 tonnes used each year to protect 3.5 million tonnes of steel or cast iron. Zinc isolates steel from aggressive products by interposing itself and forming a barrier.
In contact with the atmosphere, it becomes covered with a passivating and stable protective layer, forming a screen. This metal is distinguished by its very low corrosion rate. Another advantage is that zinc provides electrochemical protection of the steel by cathodic effect (sacrificial power) in the event of damage to the coating.
What are the different methods of applying zinc?
Hot-dip galvanizing after fabrication provides very good corrosion protection for manufactured steel products of all shapes and sizes (see table). It is also the only one that allows the inside of hollow and tubular profiles to be treated.
When steel is immersed in liquid zinc, several layers of very hard iron-zinc alloy are formed, covered with a layer of zinc. Standard A 35-503 defines three classes of steel suitable for galvanizing, according to their silicon and phosphorus contents.
These determine the coating thicknesses obtained, their structure and their appearance. The hardness of the iron-zinc compounds, higher than that of steel, gives the coating a very interesting resistance to friction and abrasion,
How are galvanized parts painted?
Galvanized steel is compatible with several painting systems to satisfy aesthetic effects or increase the longevity of its protection in particularly aggressive environments.
Pre-coated galvanized sheets (pre-treated in the factory) allow direct application of paint (topcoat) in the workshop or on site. Rilsanized sheets are treated after profiling on a plasticization line using electrostatic Rilsan powder coating on a primer layer.
The prepainted galvanized sheets combine the functions of protection against corrosion and appearance finish.
Usually carried out continuously, prepainting consists of applying to the galvanized sheet a coating based on polymerizable plastic binders, before baking. There are thermosetting lacquers such as polyesters, acrylics, and thermoplastic lacquers such as organosols or vinyl plastisols, epoxy, polyvinyldene fluoride.
Certain one-component paints, paints based on binders based on PVC, PVC-acrylic combination or aqueous dispersions of acrylic or vinyl copolymers) or two-component (based on epoxy resin or polyurethane) can be applied on galvanized steel if their formulation is compatible with zinc.
Be careful, alkyd and glycerophthalic paints should be avoided even if a primer is used. Their oxidative drying forms fatty acids which react with the zinc. This reaction gives rise to hygroscopic zinc salts causing the paint to peel.
The best way to prevent corrosion is a combination of coating your metal using appropriate electroplating metal and rounding it off with painting. It’s time to stop loosing your metal ware to rust.
