When coating any substrate, the first step is to assess the condition of the surface. This could be an uncoated substrate or one that has been previously coated. In both cases, however, the surface will need to have the correct profile and be free of contaminants.
Fig 1. Moisture finds its way through
cracks in the mill scale and the steel
begins to rust
When hot rolled steel leaves the rolling mill at approximately 1000ºC (1832ºF), the steel reacts with oxygen in the atmosphere as it cools, mill scale (iron oxide) is produced. This oxide is a blue-grey layer of scale that completely covers the surface of the steel.
In time, moisture in the atmosphere will find its way through small cracks in the mill scale and the steel will begin to rust. The corrosion occurring under the mill scale will expand, pushing the mill scale away from the material, producing a surface that is unsuitable for a coating to be applied.
Determining the Degree of Rust
The amount of rusting depends upon the humidity (dampness) of the environment and length of time the steel has been exposed to it. The Elcometer 128/1 ISO 8501-1 (commonly referred to as the Swedish Rust Standard) describes four categories of “rust grades” for new steelwork as follows:
New steel work normally falls into the first two categories and sometimes into the third category. Removing the mill scale or rust defined by the Swedish Rust Standard will require the substrate to be cleaned using grinding or abrasive blasting.
Note: mill scale only occurs on new steel.
Most modern coatings are not suited to be applied to a mill scaled surface and therefore the mill scale should be removed. ISO 8501-1 outlines a variety of methods and grades of cleanliness. The surface appearance of the steel after blast cleaning and power tooling is referred to in the standard along with descriptions of the grades of visual cleanliness, together with reference pictures.
The first criteria typically specified is the cleanliness of the surface. Before any work is carried out on the substrate, the condition of the surface is assessed using a visual comparator of the four rust grades outlined in ISO 8501-1 (A, B, C, D).
Before surface preparation, we can use the Pictorial Surface Standards to compare the condition of the surface to the photographic samples in the standard and determine what method of cleaning needs to be done. There are several versions of the pictorial standards. The choice of standard depends on which standard is specified;
Determining the Degree of Pitting
Existing structures or substrates that have been left uncoated for prolonged periods may have a high degree of corrosion. Deep areas of corrosion are referred to as pits. To determine how deep the corrosion has progressed into substrate, a pit gauge such as the Elcometer 119 Pipe Pit Gauge can be used.
Determining the Substrate Thickness
It is important to note that whilst the external surface of a structure/coating may appear to be sound, corrosion and/or erosion may have been taking place on the internal surface. The material/wall thickness can be measured through the use of ultrasonic thickness gauges.