Passivation of metal tubes

Abstract: A protective passive oxide layer is formed on the inner surfaces of metal heat pipes or tubes (1) including their end caps, welds and accompanying hardware through the use of an oxygen encapsulation method. After cleaning the tube (1) and its accompanying parts, the tube is reassembled and existing gases within the tube are removed by a pump (9) thereby creating a vacuum inside the tube. The tube is then filled with pure oxygen from a source (11) and sealed. After the oxygen is sealed within the tube, the sealed tube is heated thereby forming a passive oxide layer, such as magnetite (Fe₃O₄), on the inner surface of the tube.

Description: The present invention relates to passivation of metal tubes or heat pipes, and in particular but not exclusively to methods of providing a passive oxide layer on the inner surface of such metal tubes or heat pipes for decreasing hydrogen generation rates. The use of heat pipes or heat tubes, typically of carbon steel, is common in the power generation and chemical process industries. The use of heat pipes has proved to be very efficient at transferring heat between fluids while keeping the fluids from mixing together. Due to the continued use of the heat pipes in the heat transfer processes, corrosion on the interior surfaces of the heat pipes occurs, resulting in the formation of incondensible gases such as hydrogen. Because the gases are incondensible, they tend to build up within the heat pipe and reduce the heat pipe's ability to transfer heat thereby decreasing the efficiency and performance. It has been found that by providing an oxide layer on the interior surface of the heat pipes, particularly when made of a ferrous material such as carbon steel, the generation rates of the incondensible gases, such as hydrogen, decrease. The hydrogen production is directly related to the formation of a passive oxide layer such as magnetite (Fe₃O₄) which is formed when carbon steel is exposed to high temperature deaerated water. The reaction responsible for hydrogen generation in water-carbon steel heat pipes is summarized by the equation: 3Fe+4H₂O→Fe₃O₄+4H₂. Because incondensible hydrogen gas generation rates decrease as a passive layer is developed on the interior surface of a heat pipe, a "burn-in" method is typically used for treating and conditioning fresh carbon steel/water heat pipes. The "burn-in" process is usually conducted using high pressure water through the heat pipes at around 215°C to 300°C (419°F to 572°F). This "burn-in" process is very time consuming and can take as long as 160 hours. Other treatments such as steam, gun blueing, and hydrogen peroxide have been used to form a passive oxide layer on the interior surface of the heat pipes. Steam oxidation typically is applied at 477°c to 570°C (890°F to 1060°F) and requires a high pressure steam source. Gun blueing involves caustic chemicals and hydrogen peroxide is not effective at creating a passive surface oxide layer on carbon steel. WO-A-9 105 071 discloses the passivation of stainless steel pipes in an oxygen atmosphere after a purging gas treatment step in a specific apparatus therefor. Although several passivation processes exist for providing a passive oxide layer on the interior surface of carbon steel heat pipes, there has previously been no known process which is both economical and can be performed in a short amount of time. According to an aspect of the present invention there is provided a method of forming a passive oxide layer on an inner surface of a metal tube for reducing corrosion and thereby reduce the amount of incondensible gas formation within the tube, the method comprising: removing existing gases from the tube thereby creating a vacuum in the tube; filling the tube with oxygen; sealing the oxygen within the tube; and heating the sealed tube to form a passive oxide layer on the inner surface of the tube. The present invention also provides a method for forming a protective magnetite oxide layer (Fe₃O₄) on the interior surface of a heat pipe. The passive magnetite layer formed thereby is nearly identical to that resulting from the "burn-in" methods wherein a carbon steel heat pipe is exposed to hot water for long periods of time. The present invention utilizes an oxygen encapsulation method for producing a passive oxide layer on the inner surface of the heat pipe, wherein a passive oxide layer is formed by encapsulating pure oxygen within the heat pipe. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a schematic diagram illustrating the encapsulated oxygen passivation method according to an embodiment of the present invention; and Figure 2 is a schematic diagram of a valve and gauge assembly for evacuation and oxygen back-fill of the heat pipe or tube. Referring to Figures 1 and 2, the present invention embodied therein comprises an encapsulated oxygen passivation process wherein a heat pipe 1 (or tube) is initially cleaned in order to remove oils or other substances that could possibly react with oxygen 11 during the passivation treatment. End caps and other hardware associated with the pipe 1 are also cleaned. After cleaning, the heat pipe 1 is then assembled for treatment by the passivation process according to the preferred embodiment of the present invention. The passivation process comprises connecting the heat pipe 1 to a manifold 10 containing a vacuum pump 9, a source of oxygen gas 11, a pressure gauge 3, a vacuum gauge 2 and a vent valve 4. Through the use of the manifold 10 and its associated components, the heat pipe 1 is evacuated by the vacuum pump 9 in order to remove air and other undesirable gases from the heat pipe 1. Suitable connectors may be employed such as quick connect fittings. It is preferable to evacuate to a pressure less than 1,000 microns of Hg (0.13 Pa). After evacuation of the heat pipe 1, the heat pipe 1 is isolated from the vacuum pump 9 and back-filled with oxygen 11 under a slight positive pressure preferably 1 to 10 pounds per square inch g., PSIG (6.9 x 10³Pa to 6.9 x 10⁴Pa). After the heat pipe 1 is back-filled with oxygen 11, the heat pipe 1 is then isolated from the oxygen 11, and the manifold assembly 10 is then removed and the heat pipe 1 is quickly sealed in order to prevent the escape of the oxygen 11 encapsulated within the heat pipe 1. After sealing the heat pipe 1 and encapsulating the oxygen 11, the heat pipe 1 is then subjected to a heat treatment at a temperature preferred not to exceed 566°C (1,050°F). After heat treatment, the heat pipe 1 is then evacuated and filled with a working fluid such as water for being put into service. The oxygen encapsulation method utilized by this embodiment of the present invention for passivating heat pipes or tubes has the following advantages over other known methods of applying passive surface layers. The oxide formed with the oxygen encapsulation method is the same type as that formed during operation of the heat pipe and therefore provides optimum protective ability. Also, the oxide layer can be formed over the entire inside surface of the heat pipe tube, including welds, end caps, and fill tube. The present technique ensures that there are no chemicals that must be removed later or that can interfere with the operation of the heat pipe, and provides a much thicker oxide layer than other low temperature techniques. High pressures are not involved, as found when using steam or water. This ensures the structural integrity of the heat pipe and simplifies the process. Because only the inside surface of the heat pipe is passivated, the oxidizing atmosphere does not contact the heat treating furnace thereby preventing damage to the furnace. By encapsulating pure oxygen inside the tube, a large amount of oxygen is available for reaction to form a protective magnetite scale. If the tube were not encapsulated, the gas would expand and be forced out of the tubes. The use of air, instead of oxygen, would also make less oxygen available for reaction with the heat pipe tubes resulting in a thinner and therefore less protective oxide layer. The present method is of relatively low cost and can be accomplished with standard equipment that is used in the fabrication of heat pipes. While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention is only limited by the appended claims.

Claims: A method of forming a passive oxide layer on an inner surface of a metal tube for reducing corrosion and thereby reduce the amount of incondensible gas formation within the tube, the method comprising: removing existing gases from the tube thereby creating a vacuum in the tube; filling the tube with oxygen; sealing the oxygen within the tube; and heating the sealed tube to form a passive oxide layer on the inner surface of the tube. A method according to claim 1, wherein end caps are used to seal the tube. A method according to claim 2, wherein an oxide layer is formed over the inner surface of the tube and the end caps. A method according to claim 2 or claim 3, wherein the tube and the end caps are cleaned prior to the removing, filling and sealing steps. A method according to any one of the preceding claims, wherein a vacuum pump reducing the pressure to less than 0.13Pa is used to remove existing gases from the tube. A method according to any one of the preceding claims, wherein the tube is filled with oxygen to a pressure of 6.9 x 10³Pa to 6.9 x 10⁴Pa. A method according to any one of the preceding claims, wherein the sealed tube is heated at a temperature less than 566°C. A method according to any one of the preceding claims, wherein oxygen is provided from a pure oxygen source. A method according to any one of the preceding claims, wherein remaining gases are evacuated from the tube after the passive oxide layer is formed. A method according to any one of the preceding claims, wherein the tube is made of a ferrous material such as carbon steel whereby the passive oxide layer is magnetite.

Inventors: CARPENTER NEIL N, MARTIN DONALD T, PAUL LARRY D, CARPENTER, NEIL N., MARTIN, DONALD T., PAUL, LARRY D.
Applicants: BABCOCK & WILCOX CO, THE BABCOCK & WILCOX COMPANY
Application year: 1993
Main classification: C23C8
IPC codes: C23C8, F28F21