In circumstances where concentration changes occur in the solution in contact with the metal at the interface across which heat transfer occurs special corrosion problems can develop. Crevices, such as are formed when heat exchanger tubes are not fully expanded into a tubeplate, are particularly likely to facilitate solution concentration, and hence corrosion in many cases, under heat transfer conditions due to local boiling and evaporation.
The stress corrosion cracking of riveted seams in boilers (caustic cracking) is a particular instances. Feed waters to boilers have additives to render them less corrosive, the additions making the boiler water distinctly alkaline. If the latter can enter a riveted seam, due to inadequate caulking, evaporation of the water can lead to concentration of the caustic alkali which, if it exceeds an appropriate level and there are tensile stresses present due to misfits in fabrication, can result in stress corrosion cracking.
However, even in the absence of crevices it is possible for marked concentration changes to occur at surfaces where heat transfer conditions exist. Heat is transferred through a thin layer of superheated liquid on a metal surface, the temperature gradient rising to about 10oC as the heat transfer rate increases and the boiling point of the liquid is approached. Convection currents then transfer the heat from this thin film to the bulk liquid. When the heat flux rises to a sufficiently high value, bubbles of vapor begin to nucleate at points on the metal surface and the liquid starts to boil, with the bubbles becoming detached and, by stirring, further assisting in the transfer of heat. Further increase in the temperature of the metal surface will cause the rate of boiling to increase and bubbles become so numerous that they coalesce to form a continuous film of water vapor between the metal and bulk liquid. At that point, the vapor film acts as a thermal insulator, so that the heat flux initially falls as the temperature difference across it increases, but later rises again as the latter continues to increase and film boiling occurs. From a corrosion standpoint, the important aspects of these events are:
- due to the stirring effects involved the transport of material to and from the metal surface will differ from that which occurs in stagnant conditions
- by evaporation into a bubble, concentration of dissolved solids in the film of water between the bubble and metal surface may occur
Some of the solids dissolved in boiler water, e.g. sodium hydroxide, produce a protective film on mild steel at low concentrations but can be corrosive if sufficiently concentrated, or if the concentrated substance is not itself corrosive, it can deposit on the metal surface reducing heat transfer and raising the metal temperature so that steam, produced from water penetrating the deposit or scale, may react with the steel, if the temperature achieves values in the region of 400oC.
The effects of scales and/or corrosion products in reducing thermal conduction leading to higher temperatures in the underlying metal can lead to other modes of failure. Thus, if local hot spots are developed in a tube wall, thermal stresses will be generated and these can result in plastic deformation, apparent as bulges. (The temperature differences required to produce such effects in a mild steel tube are in the region of only 100oC.) If the local stress and temperatures are high enough and sustained then the tube may fail by creep, i.e. by the formation of intergranular voids and their coalescence into cracks. Evidence of such local hot spots is usually apparent in the microstructure of the steel, the pearlite becoming divorced or spheroidized.
There is an additional way in which thermal gradients over a metal surface immersed in an electrolyte can induced failure and this arises because of the possibility of a thermogalvanic cell being generated between the hot zone, acting as anode, and the colder regions acting cathodically. The detailed mechanisms involved in thermogalvanic cells are complex and it should not be assumed that temperature variations over a metal surface will result invariably in corrosion from such a source, which is just as well in view of the impossibility of designing any heat exchanger without some temperature gradients. However the problem does arise in some of the commonly used metals in certain environments, especially if the hot, anodic area is small in comparison with the colder, cathodic area.
The concentration of corrosive substances at metal interfaces through which heat transfer occurs can take place in circumstances that are not always easily recognized. Thermal insulation used for lagging pipe work would not usually be regarded as a particularly hazardous material from the corrosion viewpoint, but it can contain a few parts per million of chloride and the ingress of water can leach this chloride transporting it to the covered metal surface where concentration can occur due to the heat transfer. If the chloride reaches an appropriate concentration and the material being lagged is an austenitic stainless steel then stress corrosion cracking can occur, and there have been many such failures in service.
Other pages on the temperature effects on corrosion:
Acid corrosivity, Atmospheric corrosion, Christ the Redeemer, Corrosion under insulation, General effects, In seawater, Natural waters
