corrosion prevention

corrosion prevention
Due to the increasing number of surfaces available to be used on windows nowadays, it is really difficult to determine what kind of protection will provide the best results against corrosion. Some useful guidelines are explained below:


1. What is corrosion?

Corrosion or rusting refers to the erosion of metallic substances, namely Iron, due to chemical or electrochemical reactions with the corrosive agents present in the atmosphere. These agents, which can be anything from humid air to atmospheric gases released due to industrial pollution, cause erosion if they come into contact with metal surface for a prolonged period. Corrosion can occur in various forms due to the many different kinds of metals and corrosive agents that exist on earth. The difference is usually noticeable by the type of damage caused to the metal. This could be anything from the formation of crevices and depressions on the surface to the metal gaining a wrinkled and weak exterior.

1.2 Rusting of Iron
Rusting in Iron is usually recognizable by the formation of a loose reddish brown layer on the surface which consists of hydrated iron oxides.


2. How can rusting be avoided?


2.1 General measures to guard against rusting

Iron and steel can normally be protected against oxidation by:
• greasing or coating the surface with an easily removable plastic film
• coating the surface with moist-resistant paint (e.g. oil paints)
• coating the surface with stainless metals. This is done either by passing the metal through a zinc bath or by utilizing a spray gun to create an extra coating. Metals normally used to provide the exterior coating include nickel, chrome, tin and zinc
• blacking, phosphating
• cathodic protection

2.2 Common rust prevention measures related to window fittings and screws
a) galvanic treatment
To protect the raw surface of the screws and fittings, they are coated (passivation) with zinc through an electrolytic treatment. Moreover, this protection is further enhanced by giving the metal more than one protective layer. This is achieved by subjecting the metal to further passivation procedures before the first layer cools off. These extra procedures put on further coat(s) on to the surface which provides a much higher protection than just a single coat. An example could be coating with zinc-nickel-iron alloys, which usually leads to the construction blue (silver), yellow (gold) or black windows.
b) sealing the surface with organic polymer compounds
In the cathodic protection, the metals are sealed inside a film of organic polymer compounds. This film is applied after the passivation process. Furthermore, it is even possible to retain the original colour of the metal as this layer is available in a transparent shade as well as black. Hence, the protection can be strengthened many times at just a slightly higher cost.
c) sealing the surface with wax
Another possibility is to seal the metals inside a layer of wax, after it has been passivated. This method was developed by the company MACO. (Registered under: MACO Oberflächenschutz, Best.Nr. 495107 November 1999)
d) Cathodic Dip Priming / Cataphoretic Dip Priming / KTL
KTL is an organic coating system. The paint film is deposited on the cathode which is to be painted and is connected to the negative terminal of the rectifier. The prerequisite is that the paint binder is positively charged, i.e. has cathodic character.
e.) "Top Silver 1.000" zinc and aluminium flake coating method
The "Top Silver" surfaces offered by heicko provide corrosion protection in accordance with ISO 9227 NSS of up to 1,000 hours to white rust (zinc corrosion). In this case there is also no risk of any contact corrosion to many commercially tested window hardware. The ift-Rosenheim confirmed:
"Even after a continuous load of 1,000 hours, in contact with the tested hardware components, no red rust was detected on the screws." (See: Test Report 509 37933/2 from ift Rosenheim)
Zinc and aluminium flake coating is applied to the bolts by means of an aqueous dispersion. In the subsequent drying and curing process, the organic product specific coating converts into an inorganic coating which firmly adheres to the surface.



3. Test Methods - Salt spray test according to DIN 50 021-SS

The salt spray test is a commonly used anti-corrosion test method in which a salt-water solution is sprayed under constant conditions on the specimen. The test atmosphere provides a corrosive attack on the subjected parts. “The test environment is kept consistent to a street atmosphere in winter. Basically, corrosion tests are not depictions of the real stress. A conversion to a long-term resistance is not possible" (Zeschky Oberflächen – Customer Services).
The window fittings of most European manufacturers already comply with the standard corrosion protection of more than 400 hours (ISO 9227-NSS) without red rust. Moreover specialized fittings are offered which provide higher protection, which may be well over 1,000 hours to red rust.

4. Thickness of layers
The thickness of the zinc coating on its own does not ensure the corrosion resistance of a finished article (fitting, screw). The surface itself must be free of any pores, cracks or roughness to ensure that the coat glues well with surface. If this is accomplished, the resistance of the article will than depend upon the thickness of the coating layer on it. The real resistance level is than represented by the empirical value (Extract from Zeschky Oberflächen – Customer Services). Average dissolution rate in microns per year without passivation:
• Germany = 1.2
• Industrial air = 2.0-4.0
• Sea air = 1.0-2.0
• Countryside air = 0.1-1.0
Information in microns / per annum

Schliessblech source: customer photo

- Caribbean sea air
- high corrosion after 4-5 months

Environmental conditions must be considered if rust presents itself during the warranty period. The window manufacturer is therefore responsible to take sufficient appropriate precautions.

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