These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:
Galvanic protection or cathodic protection is one method often used to protect a metallic article from corrosive attack. In order to provide galvanic protection to a metallic article, the article is brought into electrical contact with an electro-chemically dissimilar metal. The metals are electro-chemically dissimilar in that one has a lower open circuit potential, also referred to as corrosion potential, than the other. The metal with the lower corrosion potential is more susceptible to corrosion (less noble) while the metal with the higher corrosion potential is less susceptible to corrosion (more noble).
When the dissimilar metals are brought into contact with one another, a galvanic couple is formed. With the addition of an electrolyte, such as saltwater, a corrosion reaction takes place in which the less noble metal acts as an anode and the more noble metal acts as a cathode. An oxidation reaction occurs at the surface of the less noble metal which supplies electrons to a reduction reaction taking place at the more noble metal, thus establishing a corrosion current between the electro-chemically dissimilar metals. As a result, the corrosion rate of the less noble metal is accelerated while the corrosion rate of the more noble metal is attenuated or completely mitigated. Therefore, the more noble metal is galvanically or cathodically protected from corrosive attack while the less noble metal is left to intentionally and sacrificially corrode.
Galvanic protection is commonly used to protect structural steels against corrosive attack. A galvanic couple is formed between the steel and an electro-chemically dissimilar zinc coating. The zinc is less noble than the steel and thus preferentially and sacrificially corrodes in the presence of a corrosive electrolyte while the underlying steel structure is protected.
It has now been unexpectedly found that a galvanic couple can be created in a single material utilizing surface treatments to bring about the necessary electro-chemical dissimilarity. In addition to providing galvanic protection against corrosion, the present invention also improves the fatigue properties and resistance to stress corrosion cracking of a metallic article.
The present invention utilizes surface treatments to locally alter the electro-chemical properties of a material and thereby create a galvanic couple to protect a particular area of a component from corrosive attack. Surface treatments, such as shot peening, burnishing, deep rolling, laser shock peening, and indenting, introduce compressive residual stress in the surface of the metallic article. The introduction of compressive residual stress is known to improve the fatigue performance and stress corrosion cracking properties of metallic materials. In addition to providing the treated material with beneficial compressive residual stress, the aforementioned surface treatments are also known to cold work the material as a result of the surface treatment operation. It is well recognized that the introduction of high amounts of cold work beneficially impacts the strength of the treated material. However, as disclosed in U.S. Pat. No. 5,826,453, it has been established that maintaining relatively low levels of cold work during the introduction of compressive residual stress improves the thermal and mechanical stability of the induced residual stress.
It has now been unexpectedly found that the resistance of a material to corrosive attack can be controlled by altering the amount of cold work contained in the material. More specifically, it has been found that, for a given metallic material, samples having a relatively high amount of cold work are more susceptible to corrosive attack and are therefore less noble than samples of the same material that have a lower amount of cold work or are not cold worked at all, such as when the samples are electro-polished. This behavior is graphically illustrated by the polarization curves shown in
This behavior is further illustrated in
Accordingly, in one embodiment, the method of the present invention utilizes the differences in corrosion potential due to different amounts of cold work in a single material to create a galvanic couple such that a specific area of the metal with higher cold work sacrificially corrodes while another area with lower cold work is protected. Referring to
The behavior observed in the test sample 100 shown in
Referring to
The method of the present invention can be used to mitigate the impact of corrosion on fatigue life while improving the resistance to fatigue failure and stress corrosion cracking of a metallic article in the following manner. The surface 146 of the lug structure 140, which is susceptible to both fatigue cracking and corrosion, is treated with a first surface enhancement to induce compressive residual stresses that offset the applied stresses, as well as any tensile residual stresses, thereby mitigating the effects of fatigue. The first surface enhancement also induces a specified, controlled amount of cold work in the surface 146. Should the metallic article 140 contain multiple areas susceptible to both corrosion and fatigue failure, the first surface enhancement may be applied to each of those areas to induce compressive residual stress with a controlled amount of cold work.
A second surface enhancement is used to treat one or more sacrificial areas 150 of the lug structure. The sacrificial areas 150, which are in electrical communication with the surface 146 treated by the first surface enhancement, are susceptible to corrosive attack but not susceptible to high-applied stresses or fatigue failure. Alternatively, the sacrificial areas 150 may be less susceptible to fatigue failure than the surface (now “protected” surface) 146. The second surface treatment induces a specified level of cold work in the sacrificial areas 150 such that the level of cold work induced by the second surface treatment is greater than the level of cold work induced by the first surface treatment at the protected surface 146 of the lug structure 140. A galvanic couple is thereby established between the areas.
The galvanic couple between the sacrificial areas 150 and the protected surface 146 is due to the different corrosion potentials associated with the levels of cold work resulting from each of the first and second surface treatments. The protected surface 146 is thereby cathodically protected from corrosive attack while the sacrificial areas 150 preferentially corrode. Further, the compressive residual stress induced in the protected surface 146 and the sacrificial areas 150 improves the resistance of the lug 140 to both fatigue failure and stress corrosion cracking.
A variety of surface treatments may be used to induce both the compressive residual stress and cold work in the component including burnishing, low plasticity burnishing, deep rolling, laser shock peening, shot peening, impact peening, pinch peening, cavitation peening, indenting or any other method capable of inducing compressive residual stress with a controlled amount of cold work.
By way of example, the fatigue and corrosion susceptible surface 146 may be treated by low plasticity burnishing or laser shock peening, thereby inducing a compressive residual stress with a relatively low amount of cold work. To produce the galvanic couple and thereby provide the necessary protection, the second, sacrificial area 150 is shot peened or impact peened to induce a comparatively higher amount of cold work than at the fatigue and corrosion susceptible surface 146. This mitigates corrosion at the corrosion susceptible surface 146 and promotes corrosion at the sacrificial area 150.
In another embodiment, a sacrificial area 150 may be located on a sacrificial feature 148, such as extra material incorporated in the design of, and electrically connected to, the structure being protected. With the application of a higher cold work surface treatment on the sacrificial feature, the sacrificial feature 148 will preferentially corrode leaving the remainder of the article protected from corrosive attack.
In another embodiment, the corrosion protection provided by the method is renewed by cleaning or otherwise removing corrosion bi-products from the sacrificial areas 150 and re-applying the second surface treatment to increase or replenish the level of cold work in the sacrificial areas 150.
The surface treatment method of the present invention can be used to treat a variety of conductive metallic structures and components subject to corrosive attack and stress related failure mechanisms such as fatigue, corrosion fatigue, and stress corrosion cracking. This includes, but is not limited to, aircraft, naval and ground-based turbines including steam turbines, aircraft structural components, aircraft landing gear and components, metallic weldments, piping and components used in nuclear, fossil fuel, steam, chemical, and gas plants, distribution piping for gases and fluids, automotive components such as gears, springs, shafts, connecting rods, and bearings, ship hulls, propellers, impellers, and shafts, rail transport components and tracks, and various other components and structures too numerous to be mentioned herein.
The previously described versions of the invention have many advantages, including providing a method for controlling and mitigating the occurrence of corrosion while simultaneously providing an improvement in the ability of a component or structure to withstand stress related failures such as fatigue and stress corrosion cracking. Previous technologies and techniques required disparate treatments to separately mitigate the effects of corrosion and fatigue.
Another advantage of the current invention is that it provides a method for galvanically or cathodically protecting a metallic article from corrosive attack without the use of dissimilar metals, an impressed current, or an external current source as is generally required for galvanic or cathodic protection. Instead, the current invention relies on a galvanic couple created by mechanical surface treatments and thus does not require the addition of any material to the protected structure nor does it require the attachment of any external material or equipment to the protected structure.
Further, the method provides a metallic article with protection against corrosive attack without the use of barrier treatments, such as painting, plating or coating the protected structure, thus eliminating the potential health and environmental risks associated with such operations. The method of the current invention is not susceptible to damage, such as cracking and chipping, and thus represents an improvement over painted, coated or plated surfaces susceptible to such damage by decreasing operation and maintenance costs for the protected article.
Another advantage of the method of the present invention is that the method can be easily incorporated into existing systems and structures, such as aging aircraft, without the associated expense of adding new materials or changing existing materials. Further, the method of the present invention can be easily incorporated into a manufacturing environment as the method can be performed as an additional machining or treatment operation.
Although the present invention has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments are possible. Therefore, the scope of the appended claims should not be limited to the description of the preferred embodiments contained herein.