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Thermally Sprayed Zinc and Aluminium Coatings

Anti Corrosive Finishing

 

Metal or thermal spraying is a technology, which protects and greatly extends the life of a wide variety of products in the most hostile environments and in situations where coatings are vital for longevity. The variety of metallised coatings is vast but can be broken down into two main categories. These include finishing coatings, such as anti-corrosion or decorative coatings, and engineering coatings such as wear resistant and thermal barrier coatings.

 

Metal spraying is carried out in a wide range of anti corrosion and engineering markets, including oil and gas, construction, petrochemical and marine. Corrosion is a major problem for these industries. There are four commonly used processes in thermal spraying; Flamespray, Arcspray, Plasma Spray and High Velocity Oxygen Fuel (HVOF), but only two of these, Flamespray and Arcspray are normally used for finishing coatings.

 

The basics

 

All methods of thermal spraying involve the projection of small molten particles onto a prepared surface where they adhere and form a continuous coating. To create the molten particles, a heat source, a spray material and an atomisation/projection method are required. Upon contact, the particles flatten onto the surface, freeze and mechanically bond, firstly onto the roughened substrate and then onto each other as the coating thickness is increased.

 

As the heat energy in the molten particles is small relative to the size of the sprayed component, the process imparts very little heat to the substrate. As the temperature increase of the coated parts is minimal, heat distortion is not normally experienced. This is a major advantage over hot-dipped galvanising.

 

In the Arcspray process, two electrically charged wires are driven and guided so that they converge at a point and form an arc. An air nozzle atomises the molten metal produced and projects it towards the work piece. The driving of the wires is typically either by air motor or electric motor and gearbox arrangement. The wires can be driven in three different ways, all which offer individual benefits.

 

Push only, where the wire is pushed from a drive unit to the pistol. Pull only, where the wire is pulled by a drive unit mounted in the pistol. Push/Pull, this method is obviously a combination of these two methods, where the wire is driven to and pulled from the pistol. Wire can typically be dispensed from portable MIG reels, coils or production packs (drums) depending on the application requirements.

 

Why use flame or arc?

 

This sounds like it should be an easily answered question, but, as with many engineering situations, a clear and precise answer is not available. In some instances, the coating properties achievable by one or the other processes does provide a simple answer. For example, arc sprayed aluminium has a bond strength that is approximately 2.5 times higher than flame sprayed aluminium. Other factors include deposit efficiency, ease of operation, safety/spray environment, changeover time, maintenance time and costs, coating finish and ease of automation.

 

The above considerations give an insight into the wide range of variables when choosing which process to use. It is worth mentioning that a matter of personal or local market preference can also be added into the decision matrix.

 

Effectiveness of metallised coatings

 

Thermal Spraying is not a new process. It has proved itself to be extremely effective

in the 90 years of its existence in all manner of applications ranging from coatings in

gas turbines to corrosion protection on park benches. As a protective system for

structural steelwork it is unsurpassed being the only system, recommended by

International and European standards EN ISO 14713 as giving greater than 20 years

to first maintenance in very aggressive environments such the marine splash zone

(category lm2) as well as all other categories.

 

Surface Engineers (Manchester).

Copyright © 2014- Surface engineers (Manchester)

September 2015 - Surface Engineers