|Title of Invention||
"A PROCESS FOR THE MANUFACTURE OF LOW POTENTIAL SACRIFICIAL ANODE FOR PROTECTION OF HIGH STRENGTH STEEL"
|Abstract||The present invention relates to a low potential sacrificial anode for protection of high strength steel comprises (i) pure aluminium in the range of 92 to 99.4 % by weight; ii) aluminium and manganese composition having 5% wt . to manganese and 95% of aluminium in the range of 0.1 wt. % to 0.6 wt.% of the aluminium; iii) 0.5 to 2% wt. of zinc; characterized in that the above composition, the impurities such as silicon, iron, copper allowable are in the range 0 to 10% wt. 0.1 to 10% wt. and 0.1 to 5% wt. respectively.|
|Full Text||FIELD OF INVENTION
The present invention relates to a sacrificial anode for cathodic protection against corrosion, hydrogen embrittlement in seawater and a process for preparation thereof.
Fabrication and installation of offshore structures require use of various materials such as metals, non-metals and composites based on the functional needs. Metals used can be of ferrous type and also of non-ferrous type such as copper base alloys. Metals of ferrous nature, which are exposed to aggressive seawater are generally protected by sacrificial anodes or impressed current cathodic protection systems. Though impressed current cathodic protection (ICCP) systems are widely used, sacrificial anodes are also used where the ICCP systems cannot be installed and/or not effective. These spaces include seawater inlet water boxes, interior spaces and rudders and near-rudder regions.
For cathodic protection of immersed structures in seawater made from ferrous metals such as iron and steels, the sacrificial anodes used generally are magnesium, aluminum, or zinc. Magnesium alloys are used only for fresh water and low salinity water applications. Though zinc is effective as galvanic anode, due to low current though put per unit mass, due to wide availability and cost considerations these are generally replaced by aluminium alloy anodes.
Aluminum is a preferred material for this service, due to its relatively low price, low density, and high theoretical electrical capacity (due to formation of a trivalent cation). However, since pure aluminum forms a protective oxide film that results in a closed circuit potential which is insufficiently
negative to cathodically protect structural steel, zinc is generally added to aluminium in small amounts and additions upto 4.5 wt.% has been known to decrease the potential of aluminium to more negative side for satisfactory cathodic protection and for increasing the efficiency of the anode.
U.S. Patent No. 3,240,688 (March 15, 1966) describes invention of an aluminium alloy with 0.04-0.5% tin and 0.005 to 1.0% gallium as alloying elements for use as sacrificial anode. This invention indicated the importance of retaining tin element in solid solution so that activation of aluminium alloy surface can take place uniformly leading to sacrificial dissolution, thereby providing sufficient current for cathodic protection of steels.
U.S. Patent No.3,379,636 (April 23, 1968) discloses the details of invention of an aluminium alloy with varying amounts of indium and gallium. The inventors claim that the alloys made exhibit a very high operating potential of around -1400 mV vs. saturated calomel electrode (SCE). This alloy is suitable for use as anode materials for primary batteries and as sacrificial anode for cathodic protection of steels.
U.S. Patent No.3,721,618 (March 20, 1973) describes the development of sacrificial aluminium alloy anode with mercury, zinc, indium and bismuth as alloying elements. The alloy exhibited satisfactory performance in low chloride bearing environments and the operating potential was found to be in the range of -1020 to -1070 mV vs. SCE, when tested for use as sacrificial anode.
Bastein et. al. in a U.S. Patent No.4,173,523 (November 6, 1979) described two types of aluminum anodes, one with 0.3 to 6% Zn or Cd, 0.02 to 0.2% of Hg, 10A/m2 at operating potential of -900 mV vs SCE while the latter
o can provide 6 A/m offering long term protection in seawater.
U.S. Patent No. 4,740,355 (April 26, 1988) by Linder et al. discloses a method of manufacturing sacrificial anodes by alloying commercial aluminium having an iron content of upto 0.5% by weight with 0.01-0.5% by weight of manganese and preferably 3.5-6% by weight of zinc and 0.01-0.05% by weight of indium. The inventors claim that the aluminium alloy described in the patent give an operating potential of -1090 to -1118 mv VS, SCE with an efficiency of about 82%, when tested as per DNV standard.
U.S. Patent No.4,885,045 (December 5, 1989, describes a method of making sacrificial aluminium anodes with varying compositions for high performance and reliability in a wide range of environmental conditions. The alloys developed with varying percentages of Zn, Mg, In, Mn etc., give an operating potential in the range of -1050 to -1090 mV v.s. SCE.
Schreider and Reding in early 1960s carried out a comprehensive test program on aluminium alloys. An Al/Zn/Hg alloy anode that maintains a relatively negative potential and an electrochemical efficiency of 95% was introduced (Materials Protection, Vol.5 No.12, pp. 15, 1966 and Materials Protections, Vol.6 No.5,pp.33, 1967) . Concurrently an aluminium anode with zinc and Indium was
also developed by Sakano et.al (Materials Protection, Vol.6,No.5, 1967, p.45) which is claimed to exhibit an efficiency of 90%. Due to stringent environment restrictions, the Hg containing alloy is no longer employed in service and Al/Zn/In anodes are extensively used as sacrificial anodes for protecting ship hulls and other submerged off shore structures.
U.S. Patent No.4,619,557 (October 28, 1986) by Salama et al. describes a flame sprayed aluminum coating of high strength steel components offering excellent seawater corrosion protection, increased fatigue life and uniform low level cathodic protection that avoids hydrogen embrittlement.
This system has the drawback in that they cannot be implemented for complex structures, fabricated out of high strength steels requiring high level of cathodic protection due to functional requirement of the structure, under consideration.
Yet another U.S. Patent No.4,684,447 (August 4, 1987) by Murali, et al. discloses a method for providing a layered electroplated aluminum base coating on the substrate to which a flame sprayed aluminum coating may adhere without the need for a roughened surface on the substrate.
Benedict in U.S. Patent No.4,941,775 (July 17, 1990) details a corrosion protection system for protecting critical parts of an offshore structure, which is made cathodic (by protective coating) to the rest of non-critical parts representing a large portion of the structure. The non-critical parts being active to the coated critical parts provide the required current ensuring corrosion protection to the critical parts. The inventors also
claim that the risk of hydrogen embrittlement of certain high strength steels, used in the critical components of offshore marine structures is greatly reduced or even completely eliminated.
The drawback of this technique is that it cannot be employed in cases where the identification of areas into critical and non-critical areas is not feasible. Also the problem due to porosity on the protective coating may lead to the deterioration.
U.S. Patent No. 5,547,560 (August 20, 1996) by Le Guyader describes a sacrificial anode, composed of an aluminum-based alloy having a gallium percentage of 0.03 to 0.20% and/or a cadmium percentage of 0.03 to 0.20% for cathodic protection of steel and alloys susceptible to corrosions in seawater. The operating potential reported in seawater claimed is in the range of -870 mV to -700 mV vs. saturated calomel electrode (SCE). This potential range, as claimed by the inventors is satisfactory for avoiding hydrogen embrittlement of high strength steels in seawater. Short term efficiency tests conducted as per DNV standards reported efficiency of 70%.
The drawback of this technique is the reduced efficiency of the anode, which results in reduced life, thereby needing frequent replacement of the anodes for protection of the structure against corrosion and hydrogen embrittlement.
BRIEF DESCRIPTION OF INVENTION
All the above referred patents and studies conducted by researchers were aimed at achieving high open circuit potential
and high efficiency, which is difficult to achieve in aluminium due to formation of a tenacious non-conducting oxide layer which inhibits the dissolution process. These reports describe methods to cast aluminium based alloys with different alloying elements for activating the surface to achieve uniform dissolution. The alloys thus developed are useful as sacrificial anodes for protection of offshore structures with a potential range varying from -950 to -1400 m V vs. SCE.
Though a high level of corrosion protection is ensured at these potentials for mild steels (commonly used offshore construction material) , high strength steels become susceptible to hydrogen embrittlement at these electronegative potentials. The hydrogen embrittlement in these steels can lead to catastrophic failures.
When steel is protected by zinc sacrificial anodes or Al-Zn-In anodes, the potential reached is generally -0.990 V v.s. SCE for former and -1.00 V vs. SCE for the latter. These potentials are sufficient to reduce the threshold for Stress Corrosion Cracking (SCC), in high strength steels. It is generally universally accepted that steel is protected around -0.8 V vs. SCE. (J. T. Reding and J.J. Newport, "The Influence of alloying elements on Aluminium anodes in seawater", (Materials Protection, December 1996, pp 15-18). In this potential range, the amount of hydrogen liberated is highly reduced as compared to that at -0.990 to -1.00 V v.s. SCE.
The more advanced design concepts utilized in the construction of high performance and longer durable vessels such as ships and submarines employ large quantities of high strength steel. Cathodic protection in the interior spaces is also affected by
the presence of high strength steels/alloys which are susceptible to SCC and hydrogen embrittlement (HE) . Also when the high strength steels are coupled to aluminium sacrificial anodes for protection against aggressive chloride attack in seawater medium, the susceptibility of these steels to SCC and HE increases (Ref: C.A. Zanis, P.W. Holdsbergand Dunn E.G. Jr., "Seawater Subcritical Cracking of HY Steel Weldments", Welding research supplement, December 1980, pp.356-63.)
Various methods have been attempted to overcome the problem of hydrogen embrittlement in cathodic protection. Some of the patents appeared in this area are also detailed here to get a overall view of the solution to this problem.
Studies carried out on structural steels such as Ni-alloy and C-Mn steels by researchers indicated that applied potentials in the range of -730 to -850 mV vs. SCE provided both acceptable SCC resistance and uniform corrosion rates. (E.Lemieux, Performance evaluation of low voltage anodes for cathodic protection, Paper No. 02016, Proc. Conf. Corrosion 2002, NACE International, 2002) . However the reported efficiency was around 55-60%.
Of the two methods available for corrosion protection, viz, the impressed current cathodic protection by DC source or galvanic current by sacrificial anodes, impressed current protection is not suitable for marine structures with complex geometry. The problem is aggravated these are made of high strength steels which are susceptible to hydrogen embrittlement. Commercially available sacrificial anodes such as Zn and Al-Zn-In are not suitable as they operate at high negative potentials (-1100 to -
1000 mV vs. SCE) than the desired range of -800 mV to -850 mV vs SCE. Also the other method of protective coatings mentioned in patents referred above suffer from various limitations.
To overcome the above referred disadvantages and to cathodically protect high strength steels, it is necessary to provide a suitable sacrificial anode with working potential in the desired range mentioned above. This apart from protecting the steel also reduces the hydrogen susceptibility. Thus there exists a need for a galvanic (sacrificial) anode which can give a working potential of around -850 mV vs. SCE, coupled with high efficiency so that long term protection is ensured.
There exists a need for the development of an aluminium alloy sacrificial anode with a working potential sufficient enough to offer protection against corrosion of high strength steel at the same time reduce the hydrogen susceptibility.
OBJECTS OF THE PRESENT INVENTION
The primary object of the present invention to propose a sacrificial anode for cathodic protection of high strength steels against corrosion hydrogen susceptibility and stress corrosion cracking and a process for preparation of such anode.
Another object of the present invention is to propose a sacrificial anode which can give a working potential of around -850 mV vs saturated calomel electrode (SCE) in seawater.
Yet another object of the present invention is to provide a sacrificial anode which has high efficiency.
A further object of the present invention is to propose a sacrificial anode which has higher life thereby eliminating the need for frequent replacement.
Still further objects and advantages of the present invention will be more apparent from the ensuing description.
At the outset of the description which follows it is to be understood that the ensuing description only illustrates a particular form of the invention. However such a particular form is only an exemplary embodiment and without intending to be understood as exemplary teaching of invention and not intended to be taken restrictively.
STATEMENT OF THE INVENTION
According to one aspect of the present invention there is provided a process for the manufacture of low potential sacrificial anode for protection of high strength steel comprising the following steps:
melting pure aluminium of weight % in the range of 90-94% of
the anode for a period of 30-45 minutes to obtain a molten
aluminium in vessel;
adding an alloy of Aluminium manganese having 5% wt. of
manganese and 95% wt. of aluminium the range of 0.1 wt.%
to 6% to the molten aluminium;
stirring the molten mixture till the alloy is fully
dissolved in the molten aluminium;
adding 0.5-2 wt.% zinc to the molten mixture;
the molten mixture is again stirred well to obtain a
homogenous molten mixture.
degassing the molten mixture in an inert atmosphere
expelling the dissolved hydrogen; and
pouring the molten alloy mixture in a cylindrical die to
obtain the sacrificial anode.
According to the other aspect of the present invention there is provided a sacrificial anode for protection of high strength steel comprising the following composition:
i) pure aluminium in the range of 92 to 99.4 % by weight;
ii) aluminium and manganese composition having 5% wt. to manganese and 95% of aluminium in the range of 0.1 wt. % to 0.6 wt.% of the aluminium;
iii) 0.5 to 2% wt. of zinc;
characterized in that the above composition, the impurities such as silicon, iron, copper allowable are in the range 0 to 10% wt. 0.1 to 10% wt. and 0.1 to 5% wt. respectively;
DETAILED DESCRIPTION OF INVENTION
The present invention relates to sacrificial anodes with low driving potential and is particularly useful for cathodic protection of high strength steels and the complex structure in marine environment. These steels are susceptible to corrosion, Stress Corrosion Cracking (SCC) and Hydrogen Embrittlement (HE) at high negative working potentials attained by conventional sacrificial anode materials, such as Zn ' and In activated aluminium alloys.
For low potential sacrificial anodes of required working
potential, it is necessary that the oxide layer on the surface of aluminium alloys are activated by the chosen alloying element and the desired working potential is achieved when coupled to a steel structure in the presence of seawater.
It has been surprisingly found that sacrificial anode with the following composition have given an open circuit potential from -850 to -900 mv vs SCE.
Zinc 0.5 to 2.00%
Manganese 0.005 to 0.30%
Silicon 0 to 0.10%
Iron 0 to 0.10%
Copper 0 to 0.05%
Tests for qualification of the anode includes a) open circuit potential measurements (at zero current), b) the closed circuit potential (working potential) when connected to the steel structure which acts as cathode (with protective current at the working potential) and c) the electrochemical efficiency, i.e., the anode capacity or the quantity of current flowing per unit weight of dissolved anode in ampere hours per kilogram.
In order to have desired working potential with protection criteria of high strength steels, the alloys have been cast within the above stated compositional range of alloying elements. The cast alloys were tested for the open circuit potential and closed circuit potential measurements (working potential) with mild steel plate in 3.5 wt. % NaCl and natural seawater. The results indicated satisfactory attainment of the potential within the desired duration of 24 hours. Also the absence of pits on the alloy surface after testing showed smooth dissolution. This
indicates that the uniform distribution of alloying elements is
responsible for activation of aluminium for dissolution.
The sacrificial aluminium anode was tested for its working potential with three different high strength steels (Ni-Cr-alloy steel) which are used for fabrication of submerged structures in naval engineering applications. The desired working potential for the protection of these high strength steels against corrosion and stress corrosion cracking were achieved within 24 hours of immersion. The anode was then fabricated with suitable insert and tested with simulated structure of these high strength steels, which indicated satisfactory performance of the alloy.
Tests for efficiency of the anode were conducted as per DNV standard RP B401 (1993) App A -"Recommended Practice for Accelerated Testing of Sacrificial Anode materials with the objective of Quality Control" This test involves exposing anode samples of diameter of 12 mm and height of 38 mm with an active surface area of 31.43 cm2 in 3.5 wt. % NaCl for a period of four days at 20°C. The current densities on the anodes was varied from 0.4 mA/ cm2 to 4.0 mA/ cm2 over this test period. The results of this test indicated average working potential of -820 mV vs. SCE and the electrochemical efficiency of around 80%.
Long-term efficiency tests have also been conducted on these anodes with high strength structural steels as cathodes with an area ratio of anode to cathode ratio varies upto 1:50 in aerated natural seawater for a period of 20 days. A copper coulometer was connected in series to determine the amount of current delivered by the anode to protect the cathode (high strength steel in this case) . The results of this long term test indicated
an electrochemical efficiency of around 82%.
The open circuit potential in seawater is -892 mV vs. SCE The working potential (Closed circuit potential) when coupled to structural steel at 1:50 anode/cathode ratio is -831 mV vs. SCE. The efficiency of the anode tested as per DNV standard is 79%.
The process of preparation of the invented sacrificial anode comprises the following steps:
(i) Pure aluminium 92-99.4 weight% of the sacrificial anode is melted in a furnace in time duration of 30-45 min. The melting point of Aluminium is 667 degrees centigrade
The concentration levels of impurity elements are Fe-0.10% max; Si.10% max; Cu-0.05% max; Mg 0.01% max.
(ii) The temperature of the molten aluminium is raised to 730-750°C in order to dissolve Al-Mn master alloy to be added in the next step. Since Manganese is the higher melting point metal this has to be dissolved at a higher temperature only. This is done to ensure better homogeneity of the melt and better dispersion of Manganese in the melt. Also the increase in temperature compensates for drop in temperature while adding Al-Mn master alloy.
(iii) Aluminium-manganese master alloy containing 5 weight % of manganese and 95 weight % aluminium is added to the molten aluminium in the range 0.1 to 6 weight %. The manganese used for making Al-Mn alloy is of electrolytic grade and of 99.995% purity.
(iv) Pure zinc is added to the molten mixture in the range 0.5
weight % to 2 weight % after complete dissolution of master alloy. After complete dissolution of Al-Mn master alloy, zinc is added. Since zinc is a low melting point metal (The melting point of Zinc is 420 degrees) , the addition of zinc will not drastically decrease the temp, of molten metal. In fact since Zinc is a low melting point metal, zinc is added when the temperature is relatively lower, viz, 700 degrees. The purity of Zinc used is 99.95% purity special high grade zinc.
(v) The resulting molten mixture is stirred well and digassed for 5-10 minutes with an inert gas such as nitrogen. Degassing is then done with nitrogen Degassing with nitrogen is done to expel the hydrogen gas dissolved in the molten aluminium. Molten Aluminium has got greater affinity to dissolve hydrogen gas, which might cause casting defects like blow holes, porosity if not removed. Hence nitrogen is used to expel dissolved hydrogen in the melt. Argon also can be used but it is highly expensive when compared to nitrogen.
(vi) The molten mixture is poured into cylindrical dies in which steel inserts are placed to obtain the sacrificial anodes in the form of cylindrical rods. The above process is further illustrated by an example.
An experimental cast was made by adopting the following procedure.
1. Aluminium of weight 2.8 kg is melted in the furnace at 690°
C and held at this temperature for around 30 minutes.
2. After all aluminium melts, temperature was raised to 730°C.
3. Aluminium - Manganese master alloy (Al-5 wt.% Mn) of weight
156 gms. is then added to the molten aluminium and stirred
4. Then 30 gms of zinc is added to the molten mixture.
5. The molten mixture was stirred well to ensure homogeneity.
6. Degassing was done for 5 minutes with Nitrogen.
7. The molten metal is poured into cylindrical die having a
steel insert to get cast sacrificial anode in form of
The anode made as per the above procedure, was found to contain the following composition in weight percentage.
Balance being Aluminium
1. A process for the manufacture of low potential sacrificial anode for protection of high strength steel
comprising the following steps:
melting pure aluminium of weight % in the range of 90-94% of the anode for a period of 30-45 minutes
to obtain a molten aluminium in vessel;
adding an alloy of Aluminium manganese having 5% wt. of manganese and 95% wt. of aluminium the
range of 0.1 wt. % to 6% to the molten aluminium;
stirring the molten mixture till the alloy is fully dissolved in the molten aluminium;
adding 0.5-2 wt.% zinc to the molten mixture;
the molten mixture is again stirred well to obtain a homogenous molten mixture;
degassing the molten mixture in an inert atmosphere expelling the dissolved hydrogen; and
pouring the molten alloy mixture in a cylindrical die to obtain the sacrificial anode.
2. The process as claimed in claim 1, wherein the manganese and zinc have a purity of 99.95%.
3. The process as claimed in claim 1, wherein the inert gas is nitrogen.
4. A low potential sacrificial anode for protection of high strength steel as claimed in claim 1,
comprising the following composition:
i) pure aluminium in the range of 92 to 99.4 % by weight;
ii) aluminium and manganese composition having 5% wt. to manganese and 95% of aluminium in the
range of 0.1 wt. % to 0.6 wt.% of the aluminium;
iii) 0.5 to 2% wt. of zinc;
characterized in that the above composition, the impurities such as silicon, iron, copper allowable are in
the range 0 to 10% wt., 0 to 10% wt. and 0.1 to 5% wt. respectively.
5. The sacrificial anode as claimed in claim 4, wherein the said electrode has an open circuit potential in the range of -850 to -900 mv versus saturated calomel electrode (SCE) in sea water at room temperature.
6. The sacrificial anode as claimed in claim 4, wherein the said electrode has a closed circuit potential or working potential of -800 to -850 mv vs. SCE measured after coupling with said cathode in aerated
3.5 wt.% NaCl.
7. The sacrificial anode as claimed in claim 4, wherein the electrode is cylindrical in shape.
|Indian Patent Application Number||2386/DEL/2004|
|PG Journal Number||11/2013|
|Date of Filing||29-Nov-2004|
|Name of Patentee||DIRECTOR GENERAL, DEFENCE RESEARCH AND DEVELOPMENT ORGANISATION|
|Applicant Address||WEST BLOCK-VIII, WING-1, SECTOR-1, R.K. PURAM, NEW DELHI - 110066|
|PCT International Classification Number||C23F 13/00|
|PCT International Application Number||N/A|
|PCT International Filing date|