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Corrosion protection coating selection / life

Reason for use:

Lookup chart for typical coating and lifetimes in given generic environments

Introduction

The selection of a coating system is dependent on the environment in which it is to operate. These environments are detailed below. This is followed by the range of systems available and a chart to indicate the typical time to first maintenance. The treatments recommended for longer lives will always protect for shorter periods and are frequently also economical for these shorter lives.

Environments

Corrosion Protection Ac Ac 002

Other Environmental Considerations

Mines

Warm Humid Conditions (water present – sometimes saline).

Specialist advice should be sought as conditions in different mines vary considerably. Zinc coatings (not aluminium or paint in coal mines) should be considered provided that the water pH is greater than 5. A sealed coating is preferred. Guidance can be sought from Environment 8 but time to first maintenance may vary widely, depending on particular conditions.

Soil

Earth, sand, rock etc.

Specialist advice is advised as the performance of the coatings will vary accordingly to the nature of the soil. Coating lives may be shortened by soluble sulphates and un-burnt coke contained in clinker and ashes. Coatings are preferably sealed. Aluminium coatings are not recommended for direct contact with alkaline clays.

Alkaline concrete away from atmosphere

Aluminium is unsuitable for direct contact with concrete due to its alkalinity and an inert barrier should be provided. This barrier is not required with zinc. Zinc coatings are beneficial in areas where carbonation of the concrete may occur.

Refrigerated Surfaces

Subject to ice formation and condensation.

Sealed or unsealed coatings are generally suitable. For temperatures below – 30°C advice should be sought.

Chemicals

Sealed metallic zinc is generally suitable for chemicals in the pH range 5 -12, sealed aluminium in the pH range 4 – 9, provided the chemical does not specifically attack the coating. The effect of the coating and sealer on the chemicals should be considered, as well as the protection of the steel.

Abrasion and Impact

Additional consideration in some applications.

The resistance to abrasion, rough handling or impact by sprayed metals (sealed or unsealed) is acceptable. The coating polishes by friction. Where abrasion is critical, specialist advice should be sought.

Table of typical coatings and life to first maintenance

Corrosion Protection Ac Ac 002b

Note a: Low corrosion category C1 has no specified coatings but will typically be 5-10 times longer than C2.

Note b: Sealed coatings will typically perform better than unsealed coatings and life to first maintenance will exceed that of an unsealed coating. Sealers should generally be applied until absorption is complete. Sealants and paints should be chosen for the specific environmental conditions.

Painting of sprayed metal coatings

Painting of sprayed metal coatings is normally only required when:

01. The environment pH value is outside the range 5-12 for zinc or 4.9 for aluminium

02. The metal is subject to direct chemical attack

03. The desired finish can only be obtained by paint

04. Additional abrasion resistance is required. Generally one or two coats of paint are sufficient, except in abnormally aggressive environments. (Sealed metal spray is normally preferable).

Metal spraying provides a very good bond surface for painted coatings and can increase the life of painted coatings.

The information in this bulletin is abstracted from BS EN ISO 14713 and BS EN ISO 2063.

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