4.1.1 Corrosion prevention, part 1
Copper alloys form the basis of traditional condenser tube and tubeplate materials, typical chemical compositions of which are given in Tables 4.1 and 4.2.
Copper-based condenser tube alloys rely for their corrosion resistance on the passivating action of a protective film of corrosion product which forms when the tubes enter service. Under conditions of turbulence and excessive water velocity, this protective film can be corroded/eroded from the tube surface, exposing the bare unprotected surface and thus promoting further attack. Such unfavourable conditions can occur in condensers operating with high water velocities orunder siphonic conditions which lead to air release and greatly increased local water velocities due to turbulence. For each material, there is a threshold water velocity above which attack of this type will occur. For example, the breakdown velocities for the protective film on aluminium brass and 70/30 cupro-nickel are 2.5 m/s and 4 m/s respectively, compared with design water velocities of the order of 1.5 m/s. The most common tube failure event is corrosion/ erosion from impingement attack, which develops from a number of causes. These are dealt with under the following headings:
(a) General impingement attack.
(b) Tube inlet attack.
(c) Lodged debris.
(d) Iron oxide scale.
(e) Waterborne debris.
(f) Mussel fouling.
(a) General impingement attack
Impingement attack is characterised by an overall surface roughening and horseshoe-shaped undercut pits, the extent of which depends upon the extent of local turbulence (Fig 4.7). The attack is aggravated by water containing a high suspended-solid burden, as is the case with condensers operating in estuarine conditions where high tidal rates are experienced. The resistance to impingement attack of conventional copper materials is illustrated in Fig 4.8. They all exhibit a threshold water velocity value, and are liable to suffer from attack under conditions of high water velocity.