Metals 2

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Level 2 Diploma in Painting and Decorating

PowerPoint Metals

Unit 215: Preparing surfaces for decoration



Learning outcomes •  Know how to prepare metal surfaces ready to receive finishing

systems. •  Be able to prepare metal surfaces ready to receive finishing

systems.



Metal classification

Task

Identify the types of metals available for use in the construction industry and discuss with your group.



Metal classification •  Pure metals – Metal that does intentionally contain another

material (eg aluminium, carbon, copper, gold, iron, lead). •  Ferrous metals – Metal that contains iron (eg steel, iron). •  Non-ferrous metals – Metal that doesn’t contain iron (eg

aluminium, copper, lead, zinc and tin). •  Alloys – Mixture of two or more metals (eg mild steel, white

aluminium). Task 1.  Which metals would be most suitable for construction of

bridges? 2.  Which metals would be most suitable for door furniture?



Metals Assessment of metal surfaces must be carefully considered prior to preparing the surface. A painter and decorator must consider the following: •  Metal classification (non–ferrous and ferrous). •  Causes of corrosion and its mechanism. •  Effects of the environment on metal surfaces. •  Identify the correct preparation method. •  Recommend the most appropriate prime to ensure the best

protection.



Causes of metal defects The following are some of the causes of metal deterioration:

•  Acids and alkalis

•  Moisture

•  Grease and dirt

•  Excessive manual handling

•  Mill scale

•  Temperature change

Task

How can a painter and decorator prevent the deterioration of metals?



Environmental effects on metals

•  Salts present in the atmosphere land on surfaces. •  These salts are absorbent in nature. They absorb moisture and this

causes rusting and corrosion.

•  Ferrous sulphate (sulphur) in the atmosphere is carried to steel and becomes contaminated at the wettest point.



Corrosion of non-ferrous metals

•  Like ferrous metals, non-ferrous metals oxidize. •  An oxide layer forms as a protective layer and this prevents further

corrosion. •  The corrosion of non-ferrous metal will not weaken the substrate

therefore painting them is for aesthetic purposes (eg copper). Soon after the exposure of copper to the atmosphere, the bright copper surface takes on a dull tan tarnish. After a few years this tarnish gradually changes to green or black.



Rust Rust is another name for iron oxide, which occurs when iron or an alloy that contains iron – like steel – is exposed to oxygen and moisture for a long period of time.

Metal + oxygen + water = rust

(Fe + O2 + H2O = Hydrated ferric oxide)



Mill scale •  This is known as oxidised iron.

•  It is a thin layer of iron oxide that covers the surface of hot-rolled steel when it leaves the rolling mills.

•  It is produced by the rapid oxidization of the white hot metal when it comes into contact with cool air.

Task

Discuss a method of removing mill scale.



Discussion task In groups, describe the methods of removing corrosion from non-ferrous and ferrous metals by hand and by using mechanical aids.


Level 2 Diploma in Painting and Decorating Mechanical Shot blasting

•  Mechanical method. •  Particles of grit are blasted onto the metal

with high air pressure. •  Removes all rust and leaves the metal surface

slightly pitted, with is a better surface for the paint system to adhere to.

Flame cleaning •  Applying a hot flame to the surface. •  Causes the rust to expand at different rates to the metal. •  Eventually – through expansion and contraction – the rust can be

easily scraped off. •  En effective way of cleaning steelwork.



Mechanical Acid pickling •  Metal is immersed in a bath of hydrochloric or sulphuric acid.

•  Should be rinsed prior to painting to prevent contamination. Needle gun •  Needles or chisels mounted in

compressed air-operated guns are vibrated at speed.

•  Hammers away the rust from the metal surface.

Task Give examples of where a needle gun and acid pickling would be used in the construction industry.



By hand •  Wire brush

•  Coarse abrasives

•  Degreasing agents or de-rusting jellies

Task Discuss in groups where the above examples would be used in the construction industry.



Questions

1.  Give an example of a ferrous and non-ferrous metal.

2.  Which three elements cause the corrosion process in ferrous metals?

3.  Name a degreasing agent for cleaning a non-ferrous metal.

4.  Name two methods of removing excessive rust from a ferrous metal.



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Date post:	10-Apr-2017
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Metals 2

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