Understanding Corrosion in Shell and Tube Heat Exchangers: Types, Locations, and Control Measures

 Corrosion In Shell and Tube Heat Exchangers

Information on corrosion types, locations in heat exchangers, and control ideas are presented in the blog.

Corrosion in an exchanger involves destruction of heat exchanger surfaces (construction materials, metals, and alloys), possibly caused by working fluids due to operating con­ditions (including stresses). Like fouling, corrosion is a complex transient phenomenon, affected by many variables, with a synergistic relationship as shown in Fig. 1 Corrosion can be classified according to such mechanisms as fretting corrosion, corro­sion fatigue, and corrosion due to microorganisms. Alternatively, corrosion can be classified based on visual characteristics of the morphology of corrosion attack, as described in the following subsection. 

                                                                        FIGURE 1 Factors influencing corrosion. (From Kuppan, 2000.)

Corrosion can induce corrosion fouling as, thus adding thermal resistance in the heat flow path and reducing heat transfer, increasing fluid pressure drop and pumping power, and increasing cost due to overdesign of the exchanger. The loss of material due to corrosion may result in crevices, holes, and/or partial removal of heat transfer surfaces, resulting in loss (leakage) of heat transfer fluids, some of which may be costly. If the fluid leaks outside, it may harm the environment if the fluid is corrosive or poisonous. If it leaks to the other fluid side, it may contaminate the other fluid and deteriorate its quality. Corrosion may add extra cost to the exchanger, due to the use of expensive material, maintenance, warranty, inventory of parts, and so on. Corrosion products carried downstream of the exchanger may corrode downstream components. Finally, corrosion may result in complete failure of an exchanger or partial failure in a tube-fin exchanger, due to corrod­ing away fins, as in an automotive radiator.

There is no a single cause of corrosion and/or associated corrosion mechanisms. However, corrosion in general has clear electrochemical roots. Namely, different parts of a heat exchanger exposed to working fluids easily become polarized. The role of an electrolyte is usually taken by a working fluid (or sometimes by solid deposits or thick metal oxide scale) in the vicinity of or between parts made of different metals. If an external electrical circuit is established, metal surfaces involved take the role of either anode or cathode. Appearance of an electric current forces electrical particles (say, positively charged metal ions) to leave the metal on the anode end and enter the sur­rounding electrolyte. On the other end, a metal surface that plays the role of cathode serves as a site where electrical current escapes from the electrolyte. The presence of this mechanism opens the way for metal dissolution at the anode end of the established electrical circuit. This dissolution can be interpreted as a corrosion effect (if all other mechanisms are suppressed). This very simplified picture provides a background for many corrosion problems.

Due to the importance of corrosion in heat exchanger design and operation, we will address only the most important topics and brief description of the main corrosion types is given first.

Subsequently, corrosion mechanisms are addressed, followed by a brief discussion of each mechanism. Possible locations of corrosion in a heat exchanger are emphasized as well. Finally, the most important guidelines for corrosion prevention are enlisted.

1 Corrosion Types

Corrosion types, important for heat exchanger design and operation, are as follows: (1) uniform attack corrosion, (2) galvanic corrosion, (3) pitting corrosion, (4) stress corro­sion cracking, (5) erosion corrosion, (6) deposit corrosion, and (7) selective leaching, as categorized by Fontana and Greene (1978).                     Let us define each corrosion type briefly.

Uniform corrosion is a form of corrosion caused by a chemical or electrochemical reaction between the metal and the fluid in contact with it over the entire exposed metal surface. It occurs when the metal and fluid (e.g., water, acid, alkali) system and operating variables are reasonably homogeneous. It is usually easy to notice corroded areas attacked by uniform corrosion. All other forms of corrosion mechanisms discussed below cause localized corrosion.

Galvanic corrosion is caused by an electric potential difference between two electrically dissimilar metals in the system in the presence of an electrolyte (such as water in a heat exchanger). It occurs on the anode and does not affect the cathode (referred to as a noble metal).

Pitting corrosion is a form of localized autocatalytic corrosion due to pitting that results in holes in the metal. If anodes and cathodes rapidly interchange the sites randomly, uniform corrosion occurs, as in rusting of iron. If the anode becomes fixed on the surface, pitting corrosion takes place.

Stress corrosion is a form of corrosion that involves cracks on susceptible metals caused by the simultaneous presence of the tensile stress and a corrosive fluid medium. 

Erosion corrosion is a form of surface corrosion due to erosion of the heat transfer surface due to a high-velocity fluid with or without particulates (e.g., fluid velocity greater than 2 m/s or 6 ft/sec for water flow over an aluminum surface) and subsequent corrosion of the exposed surface.

Crevice corrosion is a form of localized physical deterioration of a metal surface in crevices or under deposits in shielded areas (i.e., in stagnant fluid flow regions), often caused by deposits of dirt and corrosion products. 

Selective leaching or dealloying is the selective removal of one metal constituent from an alloy by corrosion that leaves behind a weak structure.

2 Corrosion Locations in Heat Exchangers

Uniform (general) corrosion is not localized, and a surrounding corrosive medium affects the surface of the exposed metal prone to corrosion. Temperature, concentrations, oxi­dation, acidity, and so on, have a significant influence on the extent of this type of corrosion. Atmospheric corrosion and high-temperature gaseous corrosion are most probable in heat exchangers. This corrosion usually thins the heat transfer surface. Metals having 0.1 mm/yr surface thinning are considered excellent, those having 0.1 to 0.5 mm/yr satisfactory, and those having above about 1 mm/yr unsatisfactory for shell­-and-tube heat exchangers (Kuppan, 2000).

As opposed to general corrosion, galvanic corrosion often attacks interfaces/contacts between tubes and baffles and/or tube sheets, contact between the baffle and shell, and joint areas (either welded, brazed, soldered, or mechanically joined). The likelihood of

galvanic corrosion can easily be assessed knowing the position of the materials involved in the galvanic series summarized in Table. The metals next to each other in the galvanic series have little tendency to galvanic corrosion. In addition, metals closer to the anodic end of the galvanic series are more prone to corrosion, and the materials at the cathodic end are more stable. It can also occur in compact and other exchangers with water and other electrolytes in the circuit in which the exchanger is one of the compo­nents.

Pitting corrosion takes place when a protective surface film breaks down; these surface films are formed on the metal surface by reaction with an environment or during the surface treatment. The common metals exposed to this type of corrosion in descending order of nobility are aluminum, stainless steels, nickel, titanium, and their alloys. It is a very aggressive type of corrosion. Pitting corrosion is influenced by metallurgical and environmental factors, such as breakdown of protective coatings, inhomogeneities in the alloys, and inhomogeneities caused by joining processes. Consequently, the appearance of pitting corrosion is possible whenever such conditions are present. Pits caused by pitting corrosion are usually at places where the metal surface has surface deformities and scratches.

Stress corrosion cracking may be present at locations within the construction where the joint interaction of stress and a corrosive medium causes material deterioration. The presence of higher stress levels, increased temperature and concentration of a corrosive medium, and crack geometry may accelerate corrosion. For example, tube­-to-tube sheet expanded joints may be prone to residual stresses, as well as thin-walled expansion joints and/or U-bends. Cold working parts and U-bends in shell-and-tube heat exchangers are locations where corrosion may take place in combination with existing stress.

TABLE 1 Galvani Series'

Mg (anodic; least noble)

Zn

Fe (galvanized) 

Al 3004

Al 3003

Cast iron

SS 430 (active) 

SS 304 (active) 

Admiralty brass 

Monel 400

SS 430 (passive) 

SS 304 (passive) 

Lead

Copper

Nickel

Inconel 825

Hastelloy C

Titanium

Graphite

Platinum (cathodic; most noble)

Erosion corrosion involves solid particle or liquid droplet impingement and cavitation. In shell-and-tube heat exchangers, impingement plates must be designed to prevent this type of erosion corrosion in tubes exposed to nozzle inlet flow. Erosion corrosion is more common at the inlet end of a heat exchanger flow passage or on the tube side.

Crevice corrosion is localized corrosion and may occur at metal-to-metal or metal-to­nonmetal joints (e.g., gasketed joints), or underneath biological growth or fouling deposits. In particular, areas prone to this type of corrosion are stagnant areas and complex geometric designs with sharp edges. This type of corrosion usually starts with an infiltration of a corrosive substance into a crack and/or small opening, such as clearances between rolled tubes and tube sheets, open welds, bolt holes, nut adjacent areas, gasket areas, or contacts between plates in a plate heat exchanger. Fouling and various deposits influence corrosion at shielded areas if the combination of fluid and heat exchanger surface material is inappropriate.

Selective leaching (parting) takes place depending on the combination of alloys selected and the presence of a corrosive substance in the surrounding medium. Some typical problems encountered in heat exchanger operation are related to (1) removal of Zn from brass in stagnant waters, (2) removal of Al from aluminum brass in acidic solutions, and (3) removal of Ni in Cu—Ni alloys under conditions of high heat flux. Such removal processes are referred to as dezincification, dealuminumification, denickeli­fication, and so on.

3 Corrosion Control

Corrosion control may be categorized as corrosion prevention and protection. In general, both prevention and protection should be planned.

Uniform corrosion can be suppressed by applying adequate inhibitors, coatings, or cathodic protection. Galvanic corrosion can be reduced by selecting dissimilar materials to be as close as possible to each other on the galvanic series list for the pairs of compo­nents in the system. In addition, insulation of dissimilar metals, application of coatings, addition of inhibitors, and installation of a third metal which is anodic to both metals in the galvanic contact may be used to minimize galvanic corrosion. Pitting corrosion is difficult to control. Materials that show pitting should be avoided in heat exchanger components. Adding inhibitors does not necessarily lead to efficient mitigation of the corrosion. The best prevention of stress-corrosion cracking is an appropriate selection of material, reduction of tensile stresses in the construction, elimination of critical en­vironmental components (e.g., demineralization or degasification), cathodic protection, and addition of inhibitors. The selection of correct material less prone to erosion, making inlet flow more uniform (thus eliminating velocity spikes), and approximate maximum velocities for a working fluid may reduce erosion effects. For example, stainless steel 316 can sustain three times larger water velocity flowing inside tubes than can steel or cooper. Also, design modifications, coatings, and cathodic protection should be considered. The best prevention of crevice corrosion is a design in which the stagnation areas of the fluid flow and sharp corners are reduced to a minimum. Design should be adjusted for complete drainage, and if possible, welding should be used instead of rolling for tubes in tubesheets. Additives to an alloy, such as arsenic or tin, may reduce dezincification, thus solving the problem with selective leaching of brass.

Increased control of corrosion may be achieved through the following means.

·     Use of corrosion-resistant and clad metal (bimetal) materials

·     Use of fluids with corrosion inhibitors

·     Good design, avoiding crevices, stagnant fluid zones, upgrading materials, having uniform and optimum fluid velocities (not too high or too low in the exchanger), using solid nonabsorbent gaskets (e.g., Teflon), minimizing tensile and residual stresses in exchanger surfaces, designing for desired startups and shutdowns, and so on

·     Proper selection of construction metals from the point of proximity in the galvanic series

·     Surface coatings, surface treatment, electrochemical protection, and so on

·     Maintaining clean exchanger surfaces (no deposits) and fluids (use a filter in the flow circuit)

·     Avoiding aluminum alloys if erosion corrosion cannot be prevented

 

To summarize, The influence of corrosion has to be taken into account not only during operation but also during design. Both prevention and protection must be included in design consid­eration. Basic information on corrosion types, locations in heat exchangers, and control ideas are presented in the blog.

Thanks for Reading!

 

References: - Dusan P 2003

                       Kuppan 2000