Corrosion is the term for a chemical or electrochemical reaction between a material, usually a metal, and its environment, which produces a deterioration of the material and its properties, e.g. rust.
Types of corrosion
A metal can be exposed to many different kinds of corrosion. The types of corrosion considered below are those most common when compact brazed heat exchangers are exposed to corrosive environments.
Pitting and crevice corrosion
In principle, pitting and crevice corrosion are the same phenomenon. Pitting may appear on exposed surfaces, for example attacking stainless steel if the passive layer is damaged. The passive layer is a protective surface film that is formed spontaneously when stainless steel 316 is exposed to air. The attack can be sudden, and may quickly cause leakage. Crevices may occur in welds that fail to penetrate, in flange joints and under deposits on the steel surface.
General corrosion is a deterioration distributed more or less uniformly over a surface. This type of corrosion is more predictable than pitting; if a device has corroded 0.1 mm in one year, it will very probably corrode 0.2 mm in two years.
Corrosion in BPHEs
When the BPHE is exposed to the recommended environment, there should be no corrosion problems. Neither stainless steel 316 nor copper corrodes easily. However, if BPHEs from the standard range are exposed to an unfavorable environment, corrosion can attack either the stainless steel 316 or the copper brazing, as shown in Figure 8.30 a and Figure 8.30 b.
BPHEs do not resist high concentrations of chloride ions (Cl-) in an oxidizing environment, because chlorides form a galvanic cell with oxygen and the metals of the BPHE. Stainless steel, in particular, is sensitive to this kind of attack, and the result may be pitting and/or crevice corrosion. It is also important to mention that higher temperatures make chlorides more aggressive towards stainless steel. When chlorides or other halogen ions (bromides, iodides) are present in high concentrations, SWEP recommends a BPHE with channel plates made of molybdenum steel (SMO 254) or a Minex with channel plates in titanium.
When copper corrodes, it is more often degraded by general corrosion corrosion than by pitting. General corrosion will most probably attack copper exposed to ammonia (NH3) or fluids with high sulfur contents. SWEP's all-stainless BPHEs have a nickel alloy as the brazing material (instead of copper), which is resistant to high sulfur and ammonia contents. Another threat to copper is the presence of dissolved salts in the fluid that affect the BPHE. Maintaining electrical conductivity within the recommended range will minimize this source of corrosion. General corrosion may attack both copper and stainless steel in strongly acidic solutions. However, copper is the more sensitive metal in an alkaline environment.
The corrosive effect of natural water can vary considerably with its chemical composition. Water quality is of great importance in avoiding corrosion in BPHEs. City water is normally of good quality, and is used as make-up water in cooling towers.
Well water is usually fairly cold and clean, which implies that it has a low biological content (see chapter 8.6). However, the concentration of scale-forming salts (calcium and magnesium sulfates and carbonates) can sometimes be very high. Pitting corrosion may be initiated under these salt deposits.
Cooling tower water (see Figure 8.31) is circulated in an open circuit between the BPHE unit and the cooling tower. The salt content can be ten times higher than in the make-up water, which is usually city water (very clean) or well water (fairly clean). In heavily polluted areas, the water may pick up dust and/or corrosive gases, such as sulfur and nitrogen compounds, during its circulation. The net effect could be a corrosive brew that requires treatment on a regular basis. As it is an open loop, treatment is fortunately quite easy to carry out.
In river and lake surface water, the concentration of scale-forming salts is usually fairly low. However, there may be appreciable amounts of solids ranging from salts and soil particles to leaves and algae. Some type of pretreatment is usually necessary, particularly to control biological activity.
Brackish water and seawater are not recommended in standard BPHEs (stainless steel 316/copper), all-stainless BPHEs (stainless steel 316/nickel) or Mo-steel BPHEs (SMO 254/copper) because of the corrosive action of the very high chloride concentrations. However, Minex is available in titanium, which is compatible with seawater. Because titanium forms a very stable, continuous and protective oxide film, it has excellent resistance to corrosion. A damaged oxide film can generally heal itself immediately, provided traces of oxygen or water are present in the environment.
In an environment containing halogen ions (e.g. chlorides or bromides) a Mo-steel BPHE in SMO 254 or a Minex with titanium channel plates is recommended. Stable and turbulent water flow does not give corrosive substances the time needed to start the corrosion process. It is therefore important to maintain a stable water flow to avoid stagnant zones inside the BPHE. For the same reason as mentioned above, it is worth rinsing and drying the BPHE carefully before a long standby.
Bleed-offs and make-up water input to cooling towers must be carried out regularly, because the circulating water in an open cooling tower will be more or less contaminated with corrosive substances. When using cooling tower water, a strainer should always be installed at the inlet of the BPHE. The recommended strainer stops particles larger than 1 mm, which correspond to a mesh size of 16-20 mesh (depending on the wire diameter). Smaller particles will pass through the heat exchanger due to the high turbulence.
Alongside the advice above, salts, chlorides, pH and temperature, etc., must be kept within the recommended ranges to avoid BPHE corrosion. For more specific information see Appendix A.