Editing Corrosion (section)

==== Glass corrosion tests ====
[[File:Spidergraph ChemDurab.png|thumb|Effect of addition of a certain glass component on the chemical durability against water corrosion of a specific base glass (corrosion test ISO 719).<ref>[http://glassproperties.com/chemical_durability/ Calculation of the Chemical Durability (Hydrolytic Class) of Glasses] {{Webarchive|url=https://web.archive.org/web/20071105084232/http://www.glassproperties.com/chemical_durability/ |date=2007-11-05 }}. Glassproperties.com. Retrieved on 2012-07-15.</ref>]]
There exist numerous standardized procedures for measuring the corrosion (also called '''chemical durability''') of glasses in neutral, basic, and acidic environments, under simulated environmental conditions, in simulated body fluid, at high temperature and pressure,<ref>{{cite web |url=http://www.vscht.cz/sil/english/chemtech_ag/vht.htm |title=Vapor Hydration Testing (VHT) |archiveurl=https://web.archive.org/web/20071214030744/http://www.vscht.cz/sil/english/chemtech_ag/vht.htm |archive-date=December 14, 2007 |work=Vscht.cz |access-date=2012-07-15}}</ref> and under other conditions.

The standard procedure ISO 719<ref>{{cite web |url=https://www.iso.org/standard/4948.html |work=International Organization for Standardization |title=Procedure 719 |year=1985 |access-date=2012-07-15 |archive-date=2018-03-14 |archive-url=https://web.archive.org/web/20180314174733/https://www.iso.org/standard/4948.html |url-status=live }}</ref> describes a test of the extraction of water-soluble basic compounds under neutral conditions: 2 g of glass, particle size 300–500 μm, is kept for 60 min in 50 mL de-ionized water of grade 2 at 98&nbsp;°C; 25 mL of the obtained solution is titrated against 0.01&nbsp;mol/L [[Hydrochloric acid|HCl]] solution. The volume of HCl required for neutralization is classified according to the table below.

{| class="wikitable"
|-
! Amount of 0.01M HCl needed to neutralize extracted basic oxides, mL
! Extracted [[Sodium oxide|Na<sub>2</sub>O]]<br>equivalent, μg
! Hydrolytic<br>class
|-
| < 0.1
| < 31
| 1
|-
| 0.1-0.2
| 31-62
| 2
|-
| 0.2-0.85
| 62-264
| 3
|-
| 0.85-2.0
| 264-620
| 4
|-
| 2.0-3.5
| 620-1085
| 5
|-
| > 3.5
| > 1085
| > 5
|}
The standardized test ISO 719 is not suitable for glasses with poor or not extractable alkaline components, but which are still attacked by water; e.g., quartz glass, B<sub>2</sub>O<sub>3</sub> glass or P<sub>2</sub>O<sub>5</sub> glass.

Usual glasses are differentiated into the following classes:

* Hydrolytic class 1 (Type I): This class, which is also called neutral glass, includes [[borosilicate glass]]es (e.g., [[Duran (glass)|Duran]], [[Pyrex]], Fiolax). Glass of this class contains essential quantities of [[boron oxide]]s, [[aluminium oxide]]s and [[Alkaline-earth oxide|alkaline earth oxides]]. Through its composition neutral glass has a high resistance against temperature shocks and the highest hydrolytic resistance. Against acid and neutral solutions it shows high chemical resistance, because of its poor alkali content against alkaline solutions.
* Hydrolytic class 2 (Type II): This class usually contains sodium silicate glasses with a high hydrolytic resistance through surface finishing. Sodium silicate glass is a silicate glass, which contains alkali- and [[Alkaline-earth oxide|alkaline earth oxide]] and primarily [[sodium oxide]] and [[calcium oxide]].
* Hydrolytic class 3 (Type III): Glass of the 3rd hydrolytic class usually contains sodium silicate glasses and has a mean hydrolytic resistance, which is two times poorer than of type 1 glasses. Acid class DIN 12116 and alkali class DIN 52322 (ISO 695) are to be distinguished from the hydrolytic class DIN 12111 (ISO 719).