Role of Design in Corrosion and Corrosion Prevention

It is well known that corrosion prevention is successfully achieved by proper selection of materials and inhibitors, by acting on the environment, and by using cathodic or anodic protection, but it is mainly achieved by a proper design that starts from scratch and fundamentals. The level of knowledge is essential and two approaches can be devised. One approach makes use of highly resistant materials, and high thicknesses, redundancies and a program of maintenance are adopted. This requires large capital investments and it is the only feasible approach in situations of "absence of knowledge", where the main resources are materials and energy. The second approach relies on information, and the knowledge of a phenomenological model of the degradation related to historical data. Critical situations are avoided at the design stage in terms of both degradation and recovery, by using inexpensive materials and protecting them if necessary. It consists in scheduled inspection programs for driving the maintenance program and updating the phenomenological model. This way requires a rational use of the resource "information" but it can bring to significant savings of the "materials" and "energy" resources. During the installation phase, the mechanical deformations necessary to make connections, the welding and drilling operations may lead to changes in material microstructure, internal stresses and damage of the coatings. In the case of concrete the distribution of reinforcing bars can lead to an uneven compaction of the concrete, and the use of spacers may allow the use of the desired thickness of the concrete cover. The hydraulic test of the equipment is often made with untreated water that, if not carefully removed at the end, can lead to microbiological corrosion problems. In the start-up phase, and especially where the movement of fluids is markedly different from that during service life can lead to phenomena of sedimentation and localized corrosion. During the service life, the operating conditions may change both for the degradation of the plant itself and for modifications of the treated fluids, the environmental conditions, the operating targets, etc. During the plant shutdowns a problem that can arise is whether or not, and how, to empty the plant and the answer depends on the properties of the materials used. Several good practice rules can be adopted in the design stage. It is important to avoid water stagnation and infiltration by using correct geometries and drainages, and to prevent condensation, caused by thermal bridges and indoors environments. The crevices caused by welding spots and unwanted deposits must be avoided. As well as the percolation of a material on another in the case that the products of atmospheric corrosion of the first material can act as cathodic sites for the corrosion of the second one. Check the galvanic coupling between different materials and, if the coupling is unwanted, use insulating joints or materials with intermediate properties or interpose parts that are easily replaced. Avoid situations that make difficult the maintenance operations and the hydrodynamic conditions that can enhance corrosion phenomena. Consider the galvanic coupling between metallic materials placed in different environments.