Glass Corrosion

Glass is much more resistant to corrosion than most materials, so much so that it is easy to think of it as corrosion-proof. Glass windows after several years exposure to the elements remain clear and apparently unaffected. Glass bottles hold a wide range of liquids that would dissolve other materials. In the laboratory, reactions are carried out in glass beakers and flasks without damage to the beakers or contamination of the solutions reacting. (reference)

Glass corrosion due to broken seal

Glass corrosion due to broken seal

But, in spite of these indications that glass is indestructible by chemical attack, under certain conditions it will corrode, even dissolve. In these cases, it is important to choose the right type of glass, since some are more corrosion resistant than others. Only a few chemicals aggressively attack glass -- hydrofluoric acid, concentrated phosphoric acid (when hot, or when it contains fluorides), hot concentrated alkali solutions and superheated water. Hydrofluoric acid is the most powerful of this group; it attacks any type of silicate glass. Other acids attack only slightly; the degree of attack can be measured in laboratory tests but such corrosion is rarely significant in service for acids other than hydrofluoric and phosphoric.

Acids and alkali solutions attack glass in different ways. Alkalis attack the silica directly while acids attack the alkali in the glass.

When an alkali solution attacks a glass surface, the surface simply dissolves. This process continuously exposes a fresh surface which in turn is dissolved. As long as the supply of alkali is sufficient, this type of corrosion proceeds at a uniform rate.

Acid corrosion behaves quite differently. By dissolving the alkali in the glass composition, a porous surface is left that consists of the silica network with holes where the alkali has been removed by the acid. This porous surface slows the rate of attack since the acid must penetrate this surface layer to find alkali to dissolve.

Corrosion by water is similar to acid corrosion in that alkali is removed from the glass surface. Water corrosion acts at a much slower rate. At high temperatures, however, water corrosion can become significant. Gauge glasses for steam boilers are a case in point. These products must be protected from the superheated water by a sheet of mica or replaced on a schedule that insures that they will not be seriously weakened.

Many laboratory tests have been devised for testing corrosion resistance. Some of them aim at accelerating rate of corrosion by employing high temperatures or by grinding the glass to a specified grain size to expose more surface area to the corroding solution. After treatment for the specified time and at the specified temperature, the weight loss of the glass can be measured or the amount of alkali extracted can be determined.

Many factors influence the rate of corrosion and no laboratory test to date is capable of predicting service behavior under all conditions. Concentration and rate of agitation of the corroding solution are important factors. As corrosion progresses, the test solution becomes contaminated with components extracted from the glass; this contamination may speed or slow corrosion rate. Some glass products have a silica-rich skin, so the surface will show a different corrosion rate from the interior. A powder test on the glass from such a product will miss this surface effect completely. Comparisons between glasses from accelerated tests will be reversed sometimes at normal service temperatures.

Comparative values of resistance to acids, alkalis and water may be found in the literature, but these should be employed as guides only. Results must be checked under actual service conditions.

Destruction of glass surfaces

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.