When selecting a corrosion protection coating the overall environment at the location of the structure requires careful consideration. A structure situated in an aggressive environment will require a much higher standard of corrosion protection than one in a benign environment. The environment can affect both the steel and the coating system. Of prime importance is the effect the environment has on the corrosion of steel.
The following steps should be followed to select the most appropriate corrosion protection coating:
The NZ Building Code requires the detailing and specifying for durability of steel structures and members with a specified intended life of not less than 50 years for building structures.
A shorter life to first maintenance may be selected in conjunction with a maintenance programme which together will meet the durability provisions of NZBC Clause B2.
Where components of the structure are not accessible for maintenance after assembly the coating must achieve the specified intended life.
General atmospheric environments (macroclimate) are classified into six atmospheric corrosivity categories (C1 to C5 and CX) based on the corrosion rates of mild steel given in ISO 9223.
In addition to climatic effects, the site specific environmental effects (microclimate) need to be considered. Factors requiring consideration include whether the steel surface is shaded, in a wet location, and whether the steel is in contact with timber or concrete. The most significant microclimate effect is if the steel surface is sheltered from rain washing but exposed to the windblown marine salts as this greatly influences the corrosion rate.
The life to first maintenance available options takes into account the asset owners expectations, and the general principles of design to avoid corrosion. The ease of maintenance of the coated article if the life to first maintenance of the coated article is less than the required durability also requires consideration.
For typical internal structural (greater than 6.0mm thickness) steelwork applications (C1 and C2), hot dip galvanizing will meet the requirement of the 50 year specified intended life as required by the Building Code. For C3 applications all galvanizing to AS/NZS 4680 will meet a 25 year minimum coating life.
For external structural (greater than 6.0mm thickness) steelwork, hot dip galvanizing to AS/NZS 4680 will achieve a life to first maintenance of 25 plus years in most corrosion zones.
Perry Metal Protection offers free technical advice to assist specifiers with making the right protective coating choice for your project.