Aluminizing is a high-temperature chemical vapor deposition (CVD) process whereby aluminum vapors diffuse into the surface of the base metal, forming new metallurgical aluminide alloys.

The aluminizing process protects the base material from corrosion in elevated temperatures. Aluminizing is used extensively by industry to protect steel components and structures from heat oxidation and sealing at service temperatures up to 10,000°C, ensuring long-term protection.

Aluminizing or aluminum diffusion alloying is an economical process for inhibiting corrosion by protecting the surface of steels, stainless steels and nickel alloys operating in severe high-temperature environments. Similar to the galvanizing process, aluminum is metallurgically bonded to the steel surface, providing excellent heat reflectivity and corrosion protection.
Aluminizing or diffusion of aluminum into the surface of the steel or alloy helps slow down or stop corrosion by protecting the surface in corrosive and/or high temperature environments. It is also very effective in combating the effects of sulfidation, oxidation, and carburization.
Aluminizing has following properties:
Sulfidation resistance – steel protection from H2S, SO2, SO3 attack
Oxidation resistance – stable aluminum oxide film formed
Carburization resistance – prevents carbon diffusion into base metal,
Hydrogen permeation – diffusion rates of H2 into steel reduced
Masked surfaces of aluminized components can be welded.
For example, steel that has been coated with an aluminum-silicon alloy, known as aluminized steel, resists corrosion and heat better than the base steel material.

Aluminized steel is used industrially for high-temperature applications such as in burners or ovens. This material is also used to make pipes that will carry corrosive materials, such as steam or acids. Aluminized steel is used in everyday products such as cookware and outdoor grills, as well. Aluminized steel exhibits superior corrosion resistance compared with galvanized steel, but typically costs more. Aluminum is preferred for its lightweight, anti-corrosive and thermal conductivity properties, whereas galvanized steel is heavier and more expensive. Aluminization has superior performance compared to galvanization for resistance to atmospheric, salt spray and muffler condensate corrosion.

Literature Review Aluminizing

Aluminizing is a process through which the surface of a metallic component is coated with a layer of aluminum. Steel and its alloys are the most common metals that are aluminized for commercial applications .Aluminized steel has good formability and it can be used to make parts containing simple bends with extreme deep drawing requirements. It also has good heat reflectivity and when exposed to temperatures below 700K, aluminized steel can reflect up to 80% of the radiant heat that impinges upon it. Aluminizing plain carbon steel or steel alloys increases their resistance to oxidation at high temperatures (773-1073K) and also increases the corrosion-resistance of steel in hydrocarbons and sulphurous atmospheres. There are several techniques that have been used to obtain a layer of aluminium over a steel surface on a commercial scale. These include; electrolytic coating, cladding, pack, gas, spray (metalizing) and hot-dip aluminizing. A brief description of the different aluminizing processes is given below.

Applications of Aluminized Steel

There are several areas of application for aluminized steels mainly where high temperature strength, oxidation and corrosion resistance is critical. One such area is in exhaust pipes which consist of front exhaust pipe connecting the exhaust manifold to the catalytic converter, an intermediate exhaust pipe connecting the catalytic converter to the muffler, and a tailpipe connected to the outlet of the muffler and serving as the exhaust outlet

Hot Dip aluminizing process

Hot-dip aluminizing is one of the oldest and most widely used processes for depositing a layer of aluminium onto a steel surface. It involves four major steps:

  1. Pretreatment : It is done whereby the surface of the substrate is cleaned to remove dusts, grease and rust if present to increase adherence of the coating. It can either be done mechanically by grinding or chemically by pickling in dilute hydrochloric or sulphuric acid .
  2. Fluxing : Fluxing is done to increase wetting of the substrate by the molten metal during dipping in molten aluminuim. This step also serves for further cleaning by immersing the substrate in a molten salt flux floating on top of the molten bath before immersion into the coating bath.
  3. Coating : The substrate is dipped in the molten bath for specific duration of time. The technique used will differ according to the shape and size of substrate that is to be coated.
  4. After-treatment : This refers to wiping, air-blasting or rolling of coated steel after withdrawal from coating bath. The objective is to reduce the amount of metal adhering to the sample.

Industrial Aluminizing Process

A continuous line of hot-dip aluminizing of strips usually consists of a feeding section, furnace, and delivery section. In the feeding section, incoming strip are uncoiled and fed into the coating line at a pre-set speed under fixed tension. The furnace section consists of the preheating, annealing, cooling and coating furnaces. If chemical cleaning is done, alkaline cleaning and water rinsing tanks are included instead of the preheating furnace [A] . The cooling furnace is connected to the annealing furnace and extends to the coating bath. It is sealed by means of a snout which extends into the molten aluminum bath. In the annealing and cooling furnaces, a dry reducing atmosphere of hydrogen and nitrogen is maintained. The delivery section is equipped to rapidly cool and allow sufficient time for the coating to set before the strips come in contact with the support roll over the coating bath.

Factors That Influence The Aluminizing Process

A number of factors play a critical role in the process of hot dip aluminizing some of which influence the thickness of intermetallic layer, morphology and phase composition while others influence the surface adhesion of the coating.

  1. Formation of the inter metallic layer due to the chemical reaction between iron in the steel and aluminium in the coating.
  2. The inter-layer is affected by the dipping time and aluminizing temperature.
  3. The carbon content of the steel substrate may also have a marked effect on the growth rate and morphology of the inter-metallic layer
  4. The thickness and morphology of the inter-metallic layer may be profoundly affected by additions to the molten bath, such as silicon.
  5. The viscosity of the molten metal may influence the wetting of the substrate.
  6. The roughness of the substrate surface determines the amount of layer of molten metal that adheres to the substrate during withdrawal from molten bath.
  7. Other factors are temperature of the metal and the time during which the coating layer is still liquid and free to draw off. During this time, the coating cools and solidifies and the intermetallic layer grows by consuming the outer layer of aluminum completely converting it to an alloy.