Sacrificial pretreatment

Sacrificial pretreatment, by acting as a hydrophobic preferred contaminant to enable bonding underwater(70). 

One of the most promising innovations in recent years is the development of hydrophobic Sacrificial Pretreatment Technology (SPT), in conjunction with hydrophobic cold-cure epoxies, to enable underwater bonding(70,83). The energetically-favourable conditions for bonding are established underwater by the application of a water-repellant preferred contaminant to a cleaned or blasted steel... 

Typically, before Ni coating on Al, a zincation pretreatment of the A1 is essential to enhance the Al-Ni interfacial contact, acting as a sacrificial layer during the autocatalytic electroless nickel plating process [3], This paper focuses on the zincation treatments for electroless nickel plating by analyzing the surface morphology and the deposited Ni properties. 

The type of membrane used can also affect the sealing issue. Heavy reinforced membranes can be more difficult to seal than thinner flat designs. Also, certain membranes have sacrificial fibers in their construction. These are leached out during pretreatment to form voids that help to reduce operating voltages. These voids form channels that can provide leakage paths along which process fluids can wick out of the electrolyzer. 

In contrast to sulfur poisoning, ZnO gives no protection against chloride compounds. Zinc oxide reacts to form Zn chloride, which also has a low melting point and cause further poisoning and sintering problems. Chloride compounds in the feedstock can be reduced by guard beds of either alkalized alumina or extra, sacrificial catalyst. Today catalyst deactivation caused by poison (sulfur and chlorine) is rarely a problem in methanol synthesis, because poisonous compounds are effectively removed in the feedstock pretreatments. 

Finally, mention should be made of the concept of a sacrificial primer which enables metals to be readily bonded underwater [210,211]. From the comments in Section 2.6.2 it is obvious that when trying to apply an adhesive to a substrate underwater it will be difficult for the adhesive to completely displace the water adsorbed on the surface and establish interfacial contact with the substrate a layer of water would remain adsorbed onto the substrate and act as a weak boundary layer. The idea of the sacrificial primer is that the substrate is firstly cleaned, typically by an abrasion pretreatment, but simultaneously a primer is applied which is formulated so that it displaces any water and is more readily adsorbed onto the substrate surface. This primer, although water repellant, is also formulated so that it is compatible with the adhesive to the extent that it, in turn, is displaced and dissolved by the adhesive when it is applied. Thus, the concept of a sacrificial primer enables the adhesive to wet the substrate completely without a weak boundary layer of water being retained at the interface. The adhesive is also especially formulated to aid these processes and contains silanes which will diffuse to the interface and establish strong and stable interfacial bonds to ensure a long service life for the joint in its underwater environment [212]. 

This is achieved by applying sacrificial core material [79]. The so-called sacrificial core material is a fiber woven into the membrane by pretreatment, and it will be dissolved in operation. After dissolution the formative hollow become the channels of water and ion. Therefore, the sediment of Mg(OH)2 and Ca(OH)2 may not block the membrane, which increases the antipollution ability. On the other hand, the resistance decreases. Figure 12.19 shows the sacrificial core material used in Nation and Aciplex. 

---