|Title of Invention||
"A ZINC-ALUMINIUM PSEUDO-ALLOY COATING TO ENHANCE THE CORROSION RESISTANCE OF STEEL STRUCTURES"
|Abstract||The present invention relates to a Zinc-Aluminium pseudo-alloy coating to enhance the corrosion resistance of steel structures. The coating consists of pseudo-alloys of Zinc and Aluminium where the total weights of Zinc and Aluminium are maintained close to 73 percent and 27 percent respectively, which has been tested to provide the best protection for the steel structure underneath. A convenient way of obtaining this is by the use of a Twin Arc Spraying method. For this, a Twin Arc Spray machine is used, fed by a Zinc wire and an Aluminium wire, preferably each of a diameter between 1.6 mm and 2.0 mm.|
|Full Text||The present invention relates to a Zinc-Aluminium (ZINCAL) coating system to enhance corrosion resistance of steel structure.
The problem of the corrosion of steel structures has become very serious in recent times. This is due to the general degradation of the environment in industrial localities due to increased pollution. Thus, the life cycle of the galvanized steel structures has been considerably reduced resulting in increased maintenance cost.
The conventional method of providing cathodic protection to a steel structure has proved very useful in prolonging the lifetime of steel structures. The principle of cathodic protection is to forcibly cause the potential of a metal to shift in a corrosive environment, by addition of external electric potential, to a non corrosive level. Steel has a potential of 600 mv. If steel potential can be forcibly lowered further by an external source, below 770 mv, corrosion of steel will be deterred. Spraying or coating with Zinc, which has a potential of 1200 mv, will continue to keep the steel potential low enough until all the mass of Zinc is exhausted.
There are two methods presently existing or commonly employed for cathodic protection. The first method uses Impressed Current. In this method, a potential or a current is super-imposed onto the structure to be protected by the means of an external power source. The impressed potential makes the steel structure cathodic in the system thus preventing the corrosion of steel.
In the second known method, involving Galvanic Coupling, the steel structure is made cathodic by giving a coating of a material which is anodic with respect to steel (e.g. Zn, Mg, Al), thus galvanically coupling the steel structure to the anode. The anodic
coatings can be given either by Hot Dip Galvanizing or by Thermal Spraying.
Thermal Spray coatings are usually applied by the existing single wire combustion wire spray equipment or the twin wire arc spraying equipment. The coatings are usually applied as single metal coatings on the steel structures using either of the above two process. The coatings of the Zn on steel structures are applied as per the IS standard.
To enhance the life of steel structures, certain Zn-Al alloys have been coated on the steel structures with varying amount of success. This is accomplished by Arc Spraying a wire of Al-Zn alloy, or spraying mixed (Al and Zn ) powders in the Powder Flame Spray Gun.
Arc Spraying of Zn-Al alloy wires has the limitation that the composition of Zn-Al alloy wire cannot go above 15% by weight of Al because of the brittle inter metallic phases that form. However, a much higher percentage of Aluminium composition is desirable. The Powder Flame Spray coatings are only slightly more advantageous than the conventional single metal coatings.
In the present invention, a coating of molten Zinc-Aluminium pseudo-alloys is provided by a process of Arc Spraying, employing a Twin Arc Spray Gun, using two wires, of Zinc and Aluminium respectively, where the tip of the wires melt simultaneously to obtain the pseudo-alloying of the two metals during the process of spraying through an electric arc. Pseudo-alloying refers to the mixing of molten metal droplets in flight during the spraying, where only partial alloying takes place. The coating mixture consists of phases of Zinc, Aluminium, Zn-Al alloys, and other phases like Zinc Oxide, Aluminium Oxide which may be present due to inflight reaction of the components. Zn-Al Pseudo Alloy Coatings show substantial advantage over conventional single metal coatings.
In the invented coating, the Zinc component acts as a sacrificial anode and the products of Zinc corrosion seal the porosity of the coating and also prevent further corrosion of zinc. The aluminium splat has oxide layers at its boundary by which the aluminium oxide layer at the splat boundary prevents further corrosion.
The Zn-Al coatings contain two phases, one Zn rich phase and the other Al rich phase. The coated specimens were subjected to conventional salt spray test using solution of composition as under.
CaS04 -1.3 Gm/Ltr.
MgC12 -2.6 Gm/Ltr.
MgS04 -1.7 Gm/Ltr
Nacl - 21.4 Gm/Ltr
Distilled Water - 1 Ltr.
The above salt spray test was carried out on
(i) The present invented Arc sprayed Zn-Al Pseudo Alloy (ZINCAL)
(ii) known Arc Sprayed Zn coatings, and
(iii) known Arc sprayed Zn + Arc Sprayed Zn-Al (Pseudo Alloy)
coating (ZINCAL) + Top Coat of High velocity Wire Flame Spray
Aluminium Bronze Coatings.
The salt spray test result show that the Arc Sprayed Zn-Al Pseudo Alloy (ZINCAL) coatings of process (i) show no signs of corrosion. Maximum corrosion is observed on the Aluminium-Bronze Sprayed sample. The corrosion of Zn coatings is somewhere in between Zn-Al and Aluminium Bronze Coatings. This shows also that the ZINCAL coatings are an improvement over the conventional Zinc Sprayed coating used for corrosion protection of steel structures, particularly in salt water environment.
Qualitatively the Zn-Al Pseudo Alloy (ZINCAL) coatings seem to be better than the conventional single metal coatings. The difference with the conventional zinc sprayed coatings applied for corrosion protection lies in the novel pseudo-alloy coatings of Zn and Al, in the unique ZINCAL coating here, having the composition of roughly 27% Al by weight and balance Zinc.
1. A Zinc-Aluminium pseudo-alloy for use as coating to enhance the corrosion resistance of steel structures, characterized in that the composition of Zinc and Aluminium in the pseudo-alloy are about 73 percent and 27 percent respectively by weight.
2. A process for producing a Zinc-Aluminium pseudo-alloy as claimed in Claim I, wherein the Zinc-Aluminium pseudo-alloy coating is obtained by a process of Twin Arc Spraying fed by two wires of Zinc and Aluminium respectively, each of a diameter between 1.6 mm and 2.0 mm.
|Indian Patent Application Number||501/DEL/1999|
|PG Journal Number||44/2010|
|Date of Filing||31-Mar-1999|
|Name of Patentee||METALLIZING EQUIPMENT COMPANY PVT LIMITED.|
|Applicant Address||E-101,M.I.A. PHASE-II, BASNI, JODHPUR-342005, INDIA.|
|PCT International Classification Number||C23C 4/08|
|PCT International Application Number||N/A|
|PCT International Filing date|