After the pipe surface has been prepared according to the required standards; the pipeline surface profile should be measured to ensure that the roughness meets requirement before commencing of composite repair of affected pipeline.
Surface Profile Measurement of Prepared Surface
The type and size of the abrasive used in blast cleaning have a significant effect on the profile or amplitude produced. In addition to the degree of cleanliness, surface preparation specifications need to consider ‘roughness’ relative to the coating to be applied. For example, shot abrasives are used for thin film paint coatings such as pre-fabrication primers, whereas thick or high build paint coatings and thermally sprayed metal coatings need a coarse angular surface profile to provide a mechanical key. Inadequate quality control and lack of restriction of large abrasive particle sizes for thin priming coats can lead to peaks of the blast cleaned surface not being adequately covered and may produce rust spots very quickly. The more recently used very high build coatings and thermal-sprayed metal coatings need a large surface area with a high profile in order to ensure that the adhesive bond is greater than the cohesive bond. The difference between these two examples of blast cleaned surfaces is illustrated in the three-dimensional (axonometric) diagrams obtained from a non-contact surface characterisation equipment.
The surface treatment specification therefore should describe the surface roughness required, usually as an indication of the average amplitude achieved by the blast cleaning process. Several methods have been developed to measure or assess the distance between the peaks and troughs of blast cleaned surfaces. These have included comparator panels, special dial gauges and replica tapes. To date only the comparator method is referenced as a standard. This method uses a square panel with a central hole surrounded by four segments with different grades of roughness. There is one comparator for grit blasted surfaces and one for shot blasted surfaces. See Figures 2 & 3 (surface profile comparators for grit and shot blasted surfaces). The appropriate comparator is placed against the substrate then visual and tactile comparisons are made. The comparators are referred to in ISO 8503-1 Parts 1 to 4, (BS 7079 Parts C1 to C4), ‘Preparation of Steel Substrates before the Application of Paints and Related Products – Surface Roughness Characteristics of Blast Cleaned Steel Substrates’. These standards describe the specification for the comparactors, the method of use and two methods for calibration.
The dial gauge and replica tape methods have been commonly used in the UK. For the dial gauge a calibrated needle gauge is first set to zero on a smooth surface and then the gauge is positioned at several points on the steel surface to measure the depths in the profile. Average readings are obtained. The replica tape method comprises the use of a two layer plastic film, one compressible, one 50 micron thick incompressible layer and a specially adapted flat anvil dial gauge. The compressible layer is placed on the surface of the blast cleaned steel and is rubbed with a circular ended tool until the surface has conformed to that of the steel, indicated by a uniform dark colouration. The tape is then removed and measured with the dial gauge. The maximum profile can then be calculated by subtracting the thickness of the non-compressible backing, i.e. 50 _m from the dial reading. The replica tape method is relatively easy to use especially on difficult to access surfaces of fabricated components. This method also provides a permanent record of the surface roughness. Commercially available tapes are known as ‘Testex, Press – O – Film’. A standard for this method is currently being drafted by an ISO committee. If an average profile reading is required, this can be obtained using a portable surface roughness meter that traverses a stylus over the cleaned surface for a defined distance, usually 0.8 mm. These instruments, such as the Talysurf 10, can measure either the arithmetic mean roughness (Ra) or the mean ‘peak to valley’ height (Rz or Rtm), however their short traverse length can only provide an indication of the surface roughness. More accurate measurements of surface roughness can be obtained from an instrument with a stylus traverse length of 2.5 mm, e.g. the Talysurf 3+ gauge. Whichever method is used to measure surface roughness, inevitably there will be rogue peaks that need to be taken into account. These can be defined as peaks of exceptional height in the blast cleaned surface and are not usually representative but can cause ‘rust rashing’ on primed surfaces where the peaks have projected above the primer coating.
Surface Profile Requirement Before Commencing Composite Repair
It is recommended that the surface roughness be between 60 and 80 microns before composite repair works or surface coating can begin. Once the pipe surface and roughness has been prepared to the required standards, the subsequent composite repair works should commenced within 4 hours.