WO2017034486A1

WO2017034486A1 - Aluminium alloy for sacrificial anode

Info

Publication number: WO2017034486A1
Application number: PCT/TH2015/000053
Authority: WO
Inventors: Tassawuth POJANAVARAPHAN; Wichapun Weerapakkaroon; Patchareeporn Sintoorahat; Pinai Mungsantisuk; Siriporn DAOPISET; Noparat KANJANAPRAYUT
Original assignee: Ptt Public Company Limited
Priority date: 2015-08-24
Filing date: 2015-08-24
Publication date: 2017-03-02

Abstract

An aluminum alloy for sacrificial anode comprising by weight silicon from 0.01% to 0.2%), copper 0.005%) or less, iron 0.1 % or less, manganese less than 0.1 %>, zinc from 3% to 7%, indium from 0.01 % to 0.02%, cadmium 0.002%> or less, an activator or a refiner from 0.01 % to 0.30%> and a remainder of aluminum and its impurities. The activator or a refiner comprises gallium, zinc oxide, titanium or titanium dioxide.

Description

TITLE OF THE INVENTION

ALUMINUM ALLOY FOR SACRIFICIAL ANODE

TECHNICAL FIELD OF THE INVENTION

The present invention relates to an aluminum alloy for use as sacrificial anode for cathodic corrosion protection.

BACKGROUND OF THE INVENTION

Sacrificial anodes are used to prevent corrosion of a less active material surface. The sacrificial anode has a more negative electrochemical potential than the protected metal. Electrons travel from the sacrificial anode via electropositive potential to the structure to be protected. During use the sacrificial anode corrodes instead of the protected metal by making the protected metal a cathode of electrochemical cell.

Sacrificial anode system is simple to install, easy to maintain, and does not require power source. The system is suitable for localized protection and is less likely to interfere with neighboring structures. Sacrificial anodes are suitable for low cathodic protection current requirement area (less than 500 mA). There are different types of anodes such as Zinc anode, Magnesium anode and Aluminum anode. Aluminum anodes are used primarily in subsea applications. During use, some anodes can lose their efficiency and become naturally passivated where non-conducting oxide film forms on the surface. As a result, the anode is no longer able to supply current to the protected structure. Thus, there is a need for a sacrificial anode that is more efficient.

SUMMARY OF THE INVENTION

The present invention discloses an aluminum alloy for sacrificial anode comprising by weight silicon from 0.01% to 0.2%, copper 0.005% or less, iron 0.1% or less, manganese less than 0.1%, zinc from 3% to 7%, indium from 0.01% to 0.02%, cadmium 0.002% or less, an activator or a refiner from 0.01 % to 0.30% and a remainder of aluminum and its impurities. The activator or the refiner of the aluminum alloy comprises gallium, zinc oxide, titanium or titanium dioxide. The activator or the refiner of the aluminum alloy ranges from 0.01% to 0.30 % by weight. The activator or the refiner of the aluminum alloy comprises 0.01 % to 0.30% gallium by weight. The activator or the refiner of the aluminum alloy comprises 0.01% to 0.30% zinc oxide by weight. In an embodiment, the activator or the refiner of the aluminum alloy comprises 0.01% to 0.30% titanium by weight. In another embodiment, the activator or the refiner of the aluminum alloy comprises 0.05% to 0.30% titanium dioxide by weight.

In an exemplary method, the aluminum alloy is installed by attaching and electrically connecting the aluminum alloy to a target structure within a distance which allows cathodic protection of target structure, where the aluminum alloy comprises a shape suitable for said target structure. In another method, the aluminum alloy is attached to a sled for retrofitting. Then the sled is placed near a target structure and the sled is electrically connecting to the target structure within a distance which allows cathodic protection of the target structure. The methods provide that the aluminum alloy is installed by placing the aluminum alloy in saline environment.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 illustrates a graph comparison between different Aluminum anodes.

Figure 2 illustrates optical micrographs of commercial Aluminum anode. Figure 3 illustrates surface features of the corroded commercial Aluminum anode.

Figure 4 illustrates surface features of the corroded a) Al-Zn-In-Ga, b) Al-Zn-In-Ti0₂, and c) Al-Zn-In-ZnO.

Figure 5 illustrates optical micrographs of a) Al-Zn-In alloy, b) Al-Zn-In-Ti0₂ alloy c) Al- Zn-In-ZnO alloy, and d) Al-Zn-In-Ti alloy. DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an aluminum alloy for use as sacrificial anode. Due to its low density, low cost and high current, aluminum alloy is an attractive sacrificial anodic material for protecting and extending the lifetime of structures surrounded by saline environment such as natural gas transmission pipeline, platforms, docks, steel pile, pier, tank and vessels, condenser, skimmer, heat exchanger, or any infrastructure.

Improvements to the microstructure and electrochemical properties are made to optimize the aluminum (Al) by alloying with activators and/or grain refiners such as gallium (Ga), titanium (Ti), zinc oxide (ZnO) and titanium dioxide (Ti0₂). In an embodiment, the present invention discloses an aluminum alloy for sacrificial anode comprising by weight silicon from 0.01% to 0.2%, copper 0.005%) or less, iron 0.1% or less, manganese less than 0.1 %, zinc from 3% to 7%, indium from 0.01 % to 0.02%, cadmium 0.002%o or less, an activator or a refiner from 0.01 % to 0.30%) and a remainder of aluminum and its impurities. To obtain the desired aluminum anode, aluminum ingots were cut, weighed and melted in graphite crucible at temperature of 750°C. The measured amounts of alloy elements were added and the melt was uniformly stirred prior to pouring into a cast iron mold, which has an ambient temperature condition. The cast aluminum alloys with approximate dimension of 1.0 in³ were submerged in saline environment and subjected to chemical analysis via optical emission spectroscopy, NACE TM0190 galvanic efficiency and consumption rate determinations and microstructure examinations. These tests have been carried out to assess anode efficiency.

The aluminum alloy has been tested for electronegative potential and electrochemical capacity. Electronegative potential tests the driving voltage available to protect the metal structure. Current capacity tests how many ampere-hours of protective current will be available for each- weight of anode material consumed to protect the metal structure. According to recommended practice (DNV-RP-B401 ), the accepted criteria for Aluminum-based anode in cathodic protection has a closed circuit potential of < -1.05 V and current capacity of at least 2,500 A-h/kg.

Figure 1 shows the comparison between commercial Al-anode, a reference (Al-Zn- In-Mg(NACE)), and improved aluminum anodes. From Figure 1 , it is apparent that adding activators and/or grain refiners increases the minimum current capacity comparing to the reference formulation Al-Zn-In-Mg (NACE). The formulation Al-5%Zn-0.015%In-0.08 to 0.12%TiO₂ has better current capacity and better efficiency. The current capacity of the aforementioned formulation is 2, 705 Ah/kg to 2,718 Ah/kg, which is better than the commercial Al-anode and is near the upper range of the Al-Zn-In-Mg(NACE). Moreover, efficiency is higher than both commercial Al-anode and Al-Zn-In-Mg(NACE). The commercial Al-anode has 89.0 % efficiency. The Al-Zn-In-Mg (NACE) formulation has 90% efficiency. The formulation Al-5%Zn-0.015%In-0.08 to 0.12%TiO₂ has 91.0% efficiency. Additionally, Al-Zn-In-Ti0₂ alloy shows the consumption rate of 3.22 kg/A-yr, which is 2.5% less than that of the commercial Al anode with consumption rate of 3.302 kg/A-yr. Adding 0.08 to 0.12% Ti0₂ has proven to be the most suitable concentration for alloying. Therefore, it is expected that Al-Zn-In-Ti0₂ alloy for sacrificial anode could provide superior life extension to the protected structure as compared to the commercial grade, once installed under the same environment.

Another example of a notable formulation is Al-5%Zn-0.015%In-0.2%TiO2. Al- 5%Zn-0.015%In-0.2%TiO2 has current capacity 2,666 Ah/kg to 2,670 Ah/kg and 89.5% efficiency. This formulation has current capacity and efficiency higher than commercial Al-anode. Thus, it can be inferred that adding titanium dioxide to aluminum anode improves efficiency of the aluminum anode.

Figures 2 to 5 illustrates the effects of adding the activators and/or grain refiners to the aluminum alloy anode. Figure 2 shows optical micrographs of commercial Al-anode having bulky dendritic crystal structure with grain size of about 350 - 650μιη. This is consistent with the localized corrosion pattern with large area of undissolved surfaces in Figure 3 where there are preferential attacks at grain boundaries. As shown, commercial Al-anode disintegrates in a non-uniform dissolution pattern. It is more susceptible to corrosion at boundaries of crystallites of material. As can be seen in Figure 3, the dissolution of the aluminum alloy is not uniform.

An ideal aluminum alloy for sacrificial anode should have a uniform dissolution. Thus, more ideal aluminum alloy anode has been developed. In an embodiment, the aluminum alloy has an activator or a refiner that are gallium, zinc oxide, titanium or titanium dioxide. The activator or the refiner ranges from 0.01% to 0.30% by weight.

In another embodiment, the aluminum alloy has an activator or a refiner with 0.01% to 0.30% gallium by weight.

In another embodiment, the aluminum alloy has an activator or a refiner with 0.01% to 0.3% zinc oxide by weight.

In yet another embodiment, the aluminum alloy has an activator or a refiner with 0.01% to 0.3% titanium by weight.

In a preferred embodiment, the aluminum alloy has an activator or a refiner with 0.05%) to 0.30% titanium dioxide by weight. Figure 4 shows examples of surface features of improved aluminum anodes that have been corroded. It is evident that a finer and more uniform structure induced by activators and/or grain refiners promotes a more uniform dissolution of the Al alloy. Adding grain refiners to aluminum alloy yields aluminum alloy that have controlled and restricted grain growth as illustrated by Figure 4a to 4c. Figure 4 a) shows the corroded Al-Zn-In-Ga anode where Ga ranges from 0.01% to 0.3%. Figure 4 b) shows the corroded Al-Zn-In-Ti0₂ anode where Ti0₂ ranges from 0.05% to 0.30%>. Ti0₂ are nano-sized particles and has high surface area that produces T1AI3 particles during the melt. T1AI3 is known to be an excellent nucleation site for solid phase, thus resulting in improved aluminum anode. Figure 4 c) illustrates the corroded Al-Zn-In-ZnO anode where ZnO ranges from 0.01% to 0.3%>. All three anodes show a more uniform corrosion pattern.

Figure 5 shows examples of aluminum alloys that have the added activators and/or grain refiners. Figure 5 a) illustrates optical micrographs of Al-Zn-In alloy which is used as the control alloy and gives the current capacity of 2,607 A h/kg and efficiency of 87.5%. Figure 5 b) to d) are aluminum that have been alloyed with Ti0₂, ZnO and Ti elements. The addition of Ti0₂, ZnO and Ti element give the current capacity between 2,553 and 2,718 A h/kg and efficiency of 85.5 to 91.0% Figure 5 b) illustrates optical micrographs of Al-Zn-In-Ti0₂ alloy. The highest current efficiency, 91.0%, was achieved after adding 0.08% to 0.12% Ti0₂ into the Al-Zn-In alloy. The Al-Zn-In-0.08% to 0.12% Ti0₂ has 2% higher current efficiency than the commercial Al alloy. The Al-Zn-In-0.08% to 0.12% Ti0₂ has 1% higher current efficiency than the Al-Zn-In-Mg (NACE) alloy. This can be explained by the beneficial effect of anatase Ti0₂ crystallites on creating nucleation sites for the solid phase, producing a finer uniform microstructure and improving the electrochemical properties of the Al alloy. Ti0₂, ZnO and Ti notably improve the solidification structure of Al anodes. Adding Ti0₂, ZnO and Ti converted bulky dendritic crystals into the small equiaxed crystals with grain size of about 100 - 300μηι. An example of Al-Zn-In-ZnO alloy is illustrated in Figure 5 c) shown in optical micrographs. In particular, adding Ti element results in grain size of 1 ΟΟμπι as illustrated in Figure 5 d) in optical micrographs of Al-Zn-In-Ti alloy. The small equiaxed crystals reduce the risk of preferential attack at grain boundaries and thus promote the uniform dissolution pattern on the surface.

Although the aluminum alloy for sacrificial anode are illustrated in Figures 3 and 4 as ingots, the aluminum alloy for sacrificial anode can be of any casted configuration such as slender stand-off, elongated flush mounted, bracelet etc. The aluminum alloy for sacrificial anode is casted for any shape that is suitable for the target structure to be protected. The aluminum alloy for sacrificial anode can be used, among others, for the cathodic protection of ferrous alloys, steel structures, fluid transmission and distribution pipelines, docks, marine structures, offshore platforms, jetty, terminals berths, and other structures exposed to saline environments with seawater strength at least 12% and temperature of 0 to 105°C.

To use the aluminum alloy for sacrificial anode, the aluminum alloy is placed within a distance which allows cathodic protection of target structure. The aluminum alloy can be installed by attaching and electrically connecting the aluminum alloy to a target structure. The aluminum alloy should be within a distance which allows cathodic protection of the target structure. The aluminum alloy is placed near the target structure where electrons can move from the aluminum alloy to the target structure for cathodic protection. The aluminum alloy is shaped so that it is suitable for the target structure. For example, the aluminum alloy can be in the form of a half shell or a bracelet for attaching to the outer surface of a pipeline. The target structure can be any structure that is surrounded by saline environment such as natural gas transmission pipeline, platforms, docks, steel pile, pier, tank and vessels, condenser, skimmer, heat exchanger, or any infrastructure.

Sometimes it is not practical or not possible to remove the target structure from the environment to replace an aluminum anode that is no longer working. Another method for installing the aluminum alloy is by first attaching the aluminum alloy onto a sled. The aluminum alloy can be attached, for example, through welding onto a sled or by any other means that allows attachment of the aluminum alloy to the sled. The sled having the aluminum alloy can be lowered into the water and placed near the target structure. In this case there is no need to move the target structure. The sled having the aluminum alloy is placed and is electrically connecting to the target structure within a distance which allows cathodic protection of the target structure. This method allows a new anode to be installed to continue cathodic protection of the target structure in cases where an old anode is no longer effective.

When aluminum alloy is placed in saline environment, it is expected to provide an excellent life extension for the target structure at least 25 years.

Claims

1. An aluminum alloy for sacrificial anode comprising by weight: silicon from 0.01% to 0.2%, copper 0.005%> or less, iron 0.1% or less, manganese less than 0.1 %, zinc from 3% to 7% , indium from 0.01% to 0.02%, cadmium 0.002% or less, an activator or a refiner from 0.01 % to 0.30% and a remainder of aluminum and its impurities.

2. The aluminum alloy of claim 1 , wherein the activator or the refiner comprises gallium, zinc oxide, titanium or titanium dioxide.

3. The aluminum alloy of claim 2, wherein the activator or the refiner comprises 0.01 % to 0.30% gallium by weight.

4. The aluminum alloy of claim 2, wherein the activator or the refiner comprises 0.01% to 0.30% zinc oxide by weight.

5. The aluminum alloy of claim 2, wherein the activator or the refiner comprises 0.01 % to 0.30% titanium by weight.

6. The aluminum alloy of claim 2, wherein the activator or the refiner comprises 0.05% to 0.30% titanium dioxide by weight.

7. The aluminum alloy of claim 2, wherein the activator or the refiner comprises 0.08% to 0.12% titanium dioxide by weight.

8. A method for installing the aluminum alloy of any one of the above claims comprising: attaching and electrically connecting the aluminum alloy to a target structure within a distance which allows cathodic protection of the target structure, wherein the aluminum alloy comprising a shape suitable for said target structure.

9. A method for installing the aluminum alloy of any one of the above claims comprising: attaching the aluminum alloy to a sled for retrofitting; placing the sled near a target structure; and electrically connecting the sled to the target structure within a distance which allows cathodic protection of the target structure.

10. The method of claims 8 or 9, wherein the aluminum alloy is placed in saline environment.