Concentration

Concentration of a liquid plays a big role into a life of any glass-lined vessel. The type of corrosive substance along with the temperature determines the effect of concentration. Concentrated acids are less corrosive than diluted acids, with the exception of Phosphoric Acid.

Temperature

Temperature has a strong effect on corrosion for both acids and bases. Generally, the rate of corrosion for bases double with every 10 oc increase in temperature. At pH 14 (NaOH) the maximum permissible limit is 57 oc. Time of exposure also plays significant role in process. The effect of exposure time on corrosion varies, depending upon the nature of corrosive substance. In case of acids, when the time of exposure is increased, the rate of corrosion tends to diminish. Bases, however, display a constant corrosion rate with time.

Different factors, such as the type of corrosive substance, its concentration, pH, temperature and agitational forces affect the rate of corrosion.

Substances

Gel 2200 provides a high degree of resistance to acids, bases, solvents, gases, uncondensed vapors, melted salts of acidic, neutral or anhydrous nature, at relatively high temperatures and at all concentrations.

Acids

Gel 2200 exhibits excellent resistance to all acids - organic and inorganic, oxidizing and reducing. The iso corrosion curves are established for most common acids. Reagent grade acids are used in laboratory test that produced these curves. In actual practice other factors such as velocity, phase type, chemical grade etc can affect the corrosion rate.

Only hydrofluoric acid causes rapid destruction of glasslining at all concentrations. When phosphoric acid concentration is increased, it becomes more aggressive towards glass. At 85% concentration the maximum useable temperature is 95 oc.

Bases

Bases are of more corrosive nature than acids. As concentration increases rate of corrosion increases. Also the rate of corrosion increases with increase in temperature. An increase of 10 oc doubles the rate of attack of glasslining. Therefore, it is important to be cautious while using hot alkalies.
Water

Water can cause severe corrosion which increases with water purity. Our Gel 2200 is competent enough to combat corrosion in vapor phase environment. Condensed water droplet on the colder surface of the glass in vapor phase, tend to leach out the alkali ion from the glass network. Therefore it is recommended that the vessel contents be slightly acidic. It is also recommended that unjacketed top dish be insulated to reduce condensation.

It is well known that hydrofluoric acid (HF) will completely destroy glasslined Vessel. Even at concentration as low as 20 ppm, fluorides in acid environment have a devastating effect on glass surface, specially in continuous reactions where the fluoride as HF are repeatedly replenished. Hydrofluoric acid reacts with silicon dioxide, the main ingredient in glass, destroying the SiO2 structure and producing a rough surface.
Preventing fluorides attack on glasslined surface requires constant vigilance by the users. Reactants which can contain fluoride impurities must be carefully analysed to determine the fluoride level before they are used. In particular technical grade phosphoric acid and its salts are often fluoride contaminated as are other mineral acids.

Corrosion Inhibition

Chemical reactions are sometimes so severe that they cause a rapid wear of glass lining. The use of additives to the reacting substances can inhibit this corrosion permitting the use of glass-lined equipment. When using acids, several hundred ppm of silica protect the glass lining and considerably reduce the rate of corrosion in liquid phase. Refer figure 3. The same result can be obtained in vapor stage by adding silicon oils. Generally, the higher the temperature, the greater the quantity of silica required and the more concentrated the acids are, the less silica needs to be used. In presence of fluorine, silica also has a favorable influence. Refer figure 4. With alkalies, a few hundred ppm of calcium, aluminum or zinc may reduce corrosion, especially in dilute solutions.

* Speed of attack depends on part per million (ppm) of micronised silica added to the batch solution.

Mechanical Properties

Glass has the disadvantages of brittleness and low tensile strength. One remedy is to place the glass lining under compression. This is achieved during cooling of the glassed item after firing. This is caused by the difference between the coefficients of thermal expansion of the glass and the base metal and the excellent bond between them.
If the glass steel composite is subjected to a mechanical strain due to handing, mechanical or thermal shock, the compressive stress must not be overcome before putting the glass into tension, causing it to fail. Therefore, the residual compressive stress in the glass acts as an effective shock absorber.

Abrasion

Abrasion of the glass lining is simply a wearing away of the glass by abrasive solids in the reactor. It is characterized by a loss of fire polish and in severe cases, a rough sandpaper-like finish.

Experience has shown that failure due to abrasion alone is very uncommon. In combination with acid corrosion, however, failure can be quite severe: abrasion weakens the silica net-work mechanically, allowing acid corrosion to accelerate.

Gel 2200 has good resistance to abrasion as well as better physical characteristics such as mechanical shock, impact resistance, thermal shock resistance etc.