Localized Erosion Corrosion of Aluminum Brass Heat Exchanger Inlet Tubes

Khalifa Omar Abouswa

Petroleum Research Center P. O. Box 6431 Tripoli / Libya E-mail [email protected]

Abstract: One-inch diameter Al brass (76.88 / 21.46 / 1.57) heat exchanger tube failed in less than two years of continuous service as a result of severe erosion corrosion attack at the inlet point. Quantity of 4 m³/h of seawater flow inside the heat exchanger tubes for cooling LPG light naphtha at flow rate of 2.19 m/s. The heat exchanger outlet tube did not show any type of attack as well as the external tube surface. Extensive localized erosion corrosion can be observed in the internal inlet tube surface, those pitting are surface defects and are sufficient to initiate turbulent flow, which led to erosion corrosion damage. Detailed macroscopic examination was carried out on the failed heat exchanger tube. The result of visual examination shows that the attack is sever localized erosion corrosion started at the step where the tube fitted into the elbow (localized high water velocity). The attack is confined to the first few inches of the tubing at the inlet end and grow a long the tube as a result of additional disturbed flow created by the erosion corrosion roughened surface. Degree of the attack is different in deferent regions, the morphology of localized corrosion attack tends towards horseshoe morphology, and then this morphology grows to emerge into one another leading to the observed morphology.

Key terms: erosion, heat exchanger, de-alloying, fluid velocity.

1 Introduction:

Brass has been widely used for components in heat exchanger, pumps, in various industrial and marine sectors. Brasses are susceptible to a corrosion process as dezincification and this susceptibility increases as the zinc content of the alloy increase. Common yellow brass are alloys of copper with 10-50% Zinc and often a number of other components including tin, iron, manganese, aluminum and lead, above 15% Zn dezincification may occur, dezincification is observed with the naked eye because the alloy assumes a red or copper color that contrasts with the original yellow. Stagnant conditions usually favor dezincification because of scale formation or foreign deposits settling on the metal surface.

The mechanism of dezincification consists of three steps, as follows: 1- The brass dissolves, 2- the zinc ions stay in solution, 3- the copper plates back on. Zinc is quite reactive, while copper is nobler. Dezincification my take place either at localized regions when it is referred to as plug-type, or uniformly over the whole surface, called layer type. In seawater, the corrosion rate of copper alloys is generally small, between 0.008 and 0.12 mm/y, but may in some instances be much greater since it varies with alloy composition and local conditions. Brass with a copper content approximately 70% is the most stable in seawater, if the copper content is higher than this, there is a tendency to localized corrosion particularly at the water line, and if the copper content is lower the probability of dezincification is increased. Copper and brasses are subject to erosion corrosion or impingement attack as the flow rate increases copper and brass tubes become more prone to impingement attack, erosion corrosion normally occurs under turbulent flow conditions. Many erosion corrosion failures occur because turbulent or turbulent flow conditions exist. Turbulence results in greater agitation of the liquid at the metal surface than is the case for laminar (straight line) flow. The most sever erosion corrosion problems occur under conditions of disturbed turbulent flow at sudden changes in the flow systems geometry, such as bends, heat exchanger tube inlets, valves, fittings etc. surface defects such as corrosion pits, deposits, and weld beads are sufficient to initiate erosion corrosion. The impingement of solid particles, entrained in a flowing liquid, can damage protective film leading to erosion corrosion. No sand partials were reported during operation. Aluminum brass and cupronickel offer a greater resistance to higher flow rates, but both have maximum limits that must not be exceeded or the surface film on the metal will be destroyed. Cu/ Ni generally provide higher level of impingement corrosion resistance.

Material Specifications and Working Condition are as follows: - Tube thickness is 3.0 mm - Inside diameter is 25.4 mm - Inlet temperature is 28 Cº - Outlet temperature is 40cº - Inlet pressure is 3.0 kg/cm² and the outlet pressure is 0.5 kg/cm² - Typical seawater analysis is given in table (1)

2 Investigations The objective of this study is to investigate the corroded and failed heat exchanger tube. The failed tube was subjected to the following metallurgical investigations including: 1- Visual examination. 2- Macroscopic and microscopic examination. 3- Hardness measurement. 4- Water analysis.

Results and Discussion The visual observation of the inlet and outlet tube ends of heat exchanger is shown in Fig (I). The heat exchanger outlet tube doesn’t suffer from any attack. The external tube surface doesn’t suffer from any type of corrosion.

Fig (1) general view of the inlet and out let heat exchanger tube ends

Extensive localized corrosion can be observed in the internal tube surface, those pitting are surface defects and are sufficient to initiate turbulent flow, which led to erosion corrosion damage. Detailed macroscopic examination was carried out on the received heat exchanger tube. The result of visual examination shows that the attack is sever localized pitting and erosion corrosion started at the step where the tube fitted into the elbow (localized high water velocity). The attack is confined to the first few inches of the tubing at the inlet end as can be seen in Fig (2) and grow a long the tube as a result of additional disturbed flow created by the erosion corrosion roughened surface.

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Fig (2) erosion corrosion of the Al brass inlet tube

Degree of the attack is different in deferent regions, the morphology of localized erosion corrosion attack tends towards horseshoe morphology as seen in fig (3) and then those pits grow to emerge into one another leading to the observed morphology.

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Fig (3) erosion corrosion and horse-show morphology at different magnifications

The higher magnification of the attacked area is shown in fig (4), the dark areas are small pits the attack progressed along the tube and formation of grooves.

Fig (4) the attack progressed along the tube

Rough surface preroughened by erosion corrosion and a typical wavy and rounded holes appearance impingement attack as seen in Fig (5).

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Fig (5) Rough surface, caused by erosion corrosion at different magnifications

Fig (6) shows tube cross-section, internal erosion corrosion and thickness reduction. The remainder of the tube is not corroded to any appreciable extent. Unfortunately those pitting seem to occur more often in the low brasses (lower zinc content). The growth of pits creating rough surface and acting as flow distributions resulting sever erosion corrosion damage and thickness reduction leading to tube perforation.

Fig (6) Tube cross section showing internal attack

Metallographic examination of the transverse specimens taken from the failed tube and remote from the tube-affected area the general appearances of the tube microstructures are shown in Fig (7) the structure consists of alpha grains with annealing twins.

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Figure (7) the tube microstructures

Hardness measurement of tube cross section samples of heat exchanger tube using Vickers hardness testing machine, the average measuring hardness values was found to be in the range of (80 to 85) HV. Water analysis: The chemical analysis of seawater is shown in table (1).

Conclusions: 1- Selected of Al brass was generally adequate for controlling erosion corrosion to a certain flow velocity. The severe erosion corrosion of the inlet heat exchange tube was probably due to high seawater velocity and turbulent flow. 2- Differential aeration cells or high-velocity conditions usually cause pitting. It can normally be avoided by keeping the brass surface clean at all the times and by avoiding high velocities and design geometry, which lead to impingement attack. 3- Spread pitting corrosion at heat exchanger internal tube surface may cause turbulence flow hence creating erosion corrosion, which later induces the observed channels. 4- Erosion corrosion is characterized in appearance by grooves, waves, horseshoe and rounded holes. 5- The calculated inlet seawater velocity was found to be 2.19 m/s while the maximum velocities in the range of (0.8- 1.5 m/s) have been suggested where the tube is pushed into fittings such as elbows.

Recommendations: 1- Cu / Ni (90 / 10) tubing is recommended because of its higher strength, higher thermal conductivity, ability to withstand mechanical de-scaling and its corrosion rate in seawater is very low. 2- Maximum attention shall be paid to control the ingress of solid particles within the seawater intake, cooling water etc. The presence of sand will markedly reduce the lifetime and performance of heat exchanger tubes. 3- Plastic inserts can be used to solve heat-exchanger-tube-inlet problems.

7 4- The flow system geometry should be designed to minimize any effects of disturbed flow.

References: 1- Engineering Solutions to Industrial Corrosion Problems, 7-9 June 1993, Sandefjord, Norway. 2- British Corrosion Journal, volume 7- number 3, 1982. 3- Metals Handbook Volume, second edition, J.R.Davis, 1998. 4- Corrosion Engineering, Fontana and Greene, Second edition, 1982. 5- Corrosion Engineering Handbook, Philip A. Schweitzer, 1996 6- R. Revie Uhlig corrosion handbook 7- L.L. Shreir Corrosion volume 2, Newnes-Butterworths 1876.

Table (1) chemical analysis of the used seawater

Chemical properties Mg / L Total dissolved solids 40300 Chloride 22300 Sulphate 2790 Alkalinity 17 Total hardness 7390 Conductivity 56000 micro mho/cm Suspended solids in normal conditions 3 Suspended solids in storm conditions 300

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