At the moment, a question puzzling me is why and how glass fibers degrade in water environment? As the degradation of the glass fibers occurs, its reduces the strength of them. As far as I know, the degradation occurs slower in seawater (containing ions) than in distilled water, which in my opinion indicates that process is related to dissociation of glass material.

How does this degradation occur and why is it different for seawater and distilled water? Do I understand correctly, that E-glass is slowly partly dissociating into ions in water environment? Or is it ion-exchange or some other mechanism?

Simply the glass degrades because it dissolves in the water.

First the how. There are two factors which impact this process which is the dissolution of sodium and the dissolution of silica. The dissolution of sodium occurs first as evidenced by a plot of pH vs time for a glass powder in water and proceed via electrophilic attack: $$\ce{Si-O-Na_{(glass)} + H2O_{(l)} \longrightarrow Si-O-H_{(glass)} + OH-_{(aq)} + Na+_{(aq)} }$$

The second step is the dissolution of the silica which proceeds via nucleophilic attack: $$\ce{(-O-)2Si(-O-H)2_{(glass)} + H2O_{(l)} <-> Si(OH)4_{(aq)} }$$

The solubility of the silica is, of course, low, but measurable and significant. This process at high temperature increases the solubility of the silica (hydrothermal) and is how natural quartz crystals were formed (hydrothermal vents).

Second you say that degradation occurs slower in seawater than distilled water and that, in fact, is correct. At first, one might suspect that the dissolved sodium reduces the dissolution due to a common ion effect or reduced chemical potential but is actually not the case. The operative word here is seawater. Seawater already has a large amount of dissolved silica in it from the rocks and sand that lines coastlines and ocean beds such that it is saturated. Since the second reaction is reversible the increased concentration prevents the silica from being leached appreciably faster than it can be redeposited thus reducing the corrosion of the fiber

• Thank you very much! Could You please provide some relevant references? Some sources, where I can read more on the topic? Mar 7 '17 at 18:21
• Do a search on aqueous corrosion of glass. These reactions are well established in the glass science community. Unfortunately my information comes from lecture notes rather than specific papers so I couldn't point you anywhere beyond that.
– A.K.
Mar 8 '17 at 8:20