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LIFE TIME COST
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The overall cost of protecting a steel fabrication throughout its life
depends on the cost and durability of the initial coating in the
particular environment and, on the costs and frequency of any
subsequent treatments where the required life exceeds that of the
initial coating.
In the majority of applications, hot dip galvanizing will provide a
long, maintenance-free life without any requirement for maintenance
painting.
There are ways of calculating the benefits or disadvantages of
different methods of corrosion protection. The most common method is to
calculate the Net Present Value (N.P.V.) of each method and compare the
results. This calculation takes into account the cost of borrowing
money, the initial cost of protection, subsequent maintenance costs and
the lifetime of the project.
It is frequently used by companies to measure the likely outcome of a capital investment project.
EXAMPLE
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Take the case of a steel structure that has a projected life of 25 years and for which the discount cost of capital is 5%.
Galvanize: system 1
Hot dip galvanize to BS EN ISO 1461 with a minimum average coating of 85µm on
steel of 6mm or more thick. As galvanizing to this standard has an
average life expectancy of more than 50 years in the UK, it is very
conservative to project a life of 25 years without further maintenance.
Let the cost of galvanizing be a base figure of 100 units. There are no
further maintenance costs. (NPV = 100)
Paint: system 2
A paint system consisting of cleaning followed by an undercoat and two
top coats of paint. This system has a life expectancy of 8 years and so
will need be repainted three times in 25 years. The initial cost is
slightly cheaper than hot dip galvanizing at 90 units. The cost of
repainting for the first two occasions is 45 units but goes up to 90
units for the third repaint when the original paint must be removed.
(NPV = 169)
Paint: system 3
A superior paint system consisting of blast cleaning followed by three
coats of a higher quality paint. This system has a life expectancy of
11 years and will need to be repainted twice in 25 years. The initial
cost is higher than the other paint system at 135 units. The cost of
repainting is half this value at 67.5 units. (NPV = 197.5)
Conclusion
It can be seen that over a 25-year project life the cost of a "cheaper"
paint system is almost 70% more than the cost of galvanizing. Likewise
the cost of a more "expensive" paint system is almost double that of
galvanizing. In initial, or first, cost terms hot dip galvanizing is
comparative with a good quality paint system. However, when looking at
lifetime costs, hot dip galvanizing works out to be considerably
cheaper than most other systems
Net Present Value compared
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