New glass is bright and sparkling, easy to see through and easy to clean and keep clean. Contrary to popular belief, the surface of the glass is not completely smooth. It has what glass manufacturers call 'lattice' or 'honeycomb' patterns. Under a microscope glass reveals a rougher surface made of peaks and potholes. Organic and inorganic contaminants fill these potholes and react chemically with the glass, firmly bonding to its surface. As a result, glass easily becomes stained and discolored, difficult to see through and difficult to clean and keep clean. The surface of the glass also possesses hydrophilic properties and is over time subjected to a corrosion process that will make its surface rougher and therefore its damage greater, in some cases irreversibly. (reference)
This has great implications on the property owner and any other users of glass, increased costs and efforts in maintenance, renovation or replacement, and in all cases a reduction in the expected performance.
Just as metal rusts, glass is subjected to a corrosion process caused by reactions between the glass surface and gases in the atmosphere. It is commonly associated with moisture or vapor attack through condensation, or reaction with an alkaline solution.
Glass is hydrophilic, meaning it attracts and holds moisture. All glass has a molecular layer of moisture on the surface. When this layer increases because of humidity or rainfall, it can obscure visibility and create a risk to comfort or safety. But most of all, it participates greatly to the destruction of the surface of the glass.
There are two distinct stages to the corrosion process, occurring together or separately. The first stage is aqueous corrosion, caused by moisture. It is referred to as ion exchange or alkali extraction (leaching). An ion exchange occurs between sodium ions from the glass and hydrogen ions from the corrosion solution. The remaining components of the glass are not altered, but the effective surface area in contact with the solution is increased. This increase in surface area leads to extraction or leaching of the alkali ions from the glass, leaving a silica-rich layer on the surface. As silica (SiO2) concentration in the glass goes down, surface area increases through dissolution of the glass surface. The pH of the solution in contact with the glass will greatly affect the corrosion process. A rapid pH increase will cause a rapid breakdown of the glass surface.
There are two types of aqueous corrosion, static and dynamic. Static aqueous corrosion is caused by an entrapment of moisture on the surface of the glass. In dynamic aqueous corrosion, the corrosion solution is replenished due to condensation run-off. Even a single droplet of moisture on unprotected glass can produce sufficient damage to be visible in good lighting.
The second stage of corrosion is a process of destruction of the leached surface layers of glass. Glass is resistant to most acids but is highly susceptible to attack by alkaline materials, especially a concentration of OH- ions giving a pH greater than 9.0. The result is an attack of the network forming silica-oxygen (Si-O) bonds, leading to dissolution of the glass surface.
Alkaline cleaning products are readily available and widely used, sometimes indiscriminately, in surface maintenance. Damage to the glass can also be caused by improper and abrasive cleaning methods.
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