Sealing with open pores

Another process of physical protection is the formation of an oxide layer that makes the metal passive. This procedure is used for aluminium. Aluminium is normally anodized in 10 per cent sulphuric acid with steel or copper cathodes until an oxide thickness of 10-100 pm is obtained. As the more superficial part of the oxide layer has a fairly open structure it is possible to deposit metals (cobalt, nickel, etc.) or organic pigments in the pores and seal with boiling water or with an alkaline solution. The colours after metallic deposition are due to interference effects. Chromic and oxalic acids are also used significantly as electrolyte. 

The carbon and graphite articles obtained using the above-described processes contain up to 20 to 25% by volume of open pores formed by the escape of gaseous pyrolysis products upon binder carbonization. For many applications it is essential to fill up at least some of the pores by posttreatment or to seal the surface to gases and liquids. This is accomplished with ... 

The apparent porosity, which is the ratio of the volume of the open pores to the bulk volume (expressed as a percentage), is of importance in that it gives an indication of the resistance of the refractory to attack by liquid and gaseous phases. The sealed pores — which together with the open pores constitute the true porosity -are not very important from the viewpoint of performance of the material. 

To take up water, a brick must be porous. There are two measures of water absorption one obtained after soaking the brick for 24 h in cold water and another, larger one obtained after boiling the brick for 5 h.The difference between the two values represents the so-called sealed pores (pores that are not accessible to water under normal conditions, such as wetting by rain). If all open pores are filled with water, the unfilled sealed pores provide space into which water can expand on freezing with little or no development of stress. 

Figure 16.9 Triamine-based gating strategy (a) at high pH the amine groups interact with each other and the pore wall through hydrogen bonding and the gate is open (b) at low pH the protonated ammonium ions repel each other, closing the gate (c) in the presence of bulky polyvalent counterions the pores are effectively sealed.

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