Impact of Corrosion on the Life and Performance of Shell and Tube Heat Exchangers

Shell and Tube heat exchangers are critical and complex components used in multiple industries (e.g., power, petrochemical, oil & gas, marine, etc.). These units present challenges to prevent and mitigate corrosion due to configuration and a combination of metals.

By James (Jim) Holden, PE – Engineering and Technical Sales, Director, Cortec Global Services

The shell of a shell and tube heat exchanger is normally made from carbon steel. However, the shell, tube sheets, tubes, baffles, nozzles, and channel covers of shell and tube heat exchangers can be manufactured from carbon steel, copper, copper-nickel, stainless steel, Hastelloy, Inconel, or titanium. Material selection for the various components is driven by the fluids being managed, the temperature, mechanical and thermal stress, and corrosion resistance. Even though the design is such that there is clearance between the tubes and baffles, there is line contact between the two due to tube deflection and weight of the baffle, which can lead to galvanic corrosion.

Galvanic Corrosion

Galvanic corrosion occurs when dissimilar metals are in contact in the presence of an electrolyte. The further apart on the galvanic scale the more severe the corrosion, which is also impacted by the surface ratio of the more noble metal (cathode) to the less noble metal (anode). If the surface area of the anode is equal to or smaller than the surface area of the cathode, it leads to higher current density in the anode resulting in a greater rate of corrosion.¹

Crevice Corrosion Cracking

There is also the potential for crevice corrosion cracking between the tube and tube sheet due to the tight clearance between the tube and tube sheet. This potential is increased for tubes that are welded to the tube sheet due to the welding stress. In addition to crevice corrosion cracking, there is also a potential for stress corrosion cracking especially of stainless-steel components (tube, tube sheet, baffles) when exposed to a corrosive environment with a high chloride content and high tensile stress.²

Crevice corrosion cracking can lead to stress corrosion cracking, which is the most severe form of corrosion cracking, and can lead to sudden and unexpected failures.³

While it is difficult to prevent corrosion during operation, especially crevice corrosion and resultant cracking, the impact can be minimized during shutdown periods by providing corrosion protection.

The optimum solution occurs during the design phase. It is an opportunity to consider how to minimize the impact of corrosion through geometry configuration, as well as methods that provide corrosion protection from the manufacturing stage through installation, operation, and shutdown.

Vapor Phase Corrosion Inhibitors

There are multiple ways to provide corrosion protection during the life cycle of a shell and tube heat exchanger, but this article will concentrate on the use of vapor phase corrosion inhibitors.

At various times during the life cycle of a shell and tube heat exchanger the shell and/or the tube bundle are hydrotested to demonstrate mechanical integrity. This is a perfect opportunity to introduce a vapor phase corrosion inhibitor. The inhibitor is added to the hydrotest water at between .5% and 3% by weight volume and mixed to obtain a homogeneous solution. After hydrotest, the solution is drained leaving behind a volume that is saturated with the inhibitor molecules and a molecular film adsorbed onto the surface of the metal. Normally this does not require removal prior to operation.

The other method of providing corrosion protection to the shell and/or tube side of a shell and tube heat exchanger is to fog with a vapor phase corrosion inhibitor when not in operation, such as during shipping, warehouse, and shutdown periods. The product is applied at .3 oz./ft3 – .5 oz./ft3 (.31 L/m3 – .52 L/m3) as an alternate to nitrogen. Both the hydrotest additive and the fogging product provide protection by displacing moisture from the metal surface (hydrophobic) and being adsorbed (ionic, covalent, or metallic) onto the metal surface creating a microscopic barrier film which does not impact the material properties and normally does not need to be removed prior to operation. In layperson terms, the inhibitor molecules interact with positive and negative charges on the metal surface creating a net “0” surface charge, with no free electrical charge for corrosive elements to interact with a corrosion cell that cannot initiate.

ABOUT THE AUTHOR

James (Jim) E. Holden, PE – Engineering and Technical Sales, is a Director at Cortec Global Services, which is a company that specializes in preservation programs for complex systems. He has a professional engineer license, a bachelor’s degree in Mechanical Engineering, a master’s degree in Business, and a Master Black Belt certification in Six Sigma Quality. Jim has 40 years of experience working directly in the energy field, including tours at GE, Dresser-Rand, Alstom Steam Turbine, and Westinghouse. He has spent 11 years at Cortec® addressing corrosion-related issues.