The development of thermal spray technologies as an integrated system, i.e. hardware and spray materials, has evolved to a state where thermal spray can be looked at as a viable method of corrosion control for application in pulping digesters. High Velocity Oxygen Fuel spray systems combined with new materials designed to have particularly good corrosion resistance, provide a superior barrier to solution penetration to substrates. Coating chemistries, micrographs of the corrosion samples and their description are presented.
Corrosion in pulp digesters, both batch and continuous have been an ongoing problem in the pulp and paper industry. The corrosive effect of the chemicals used in the pulping process leads to vessel wall wastage and attack at welded areas. The abrasive/ erosive effect caused by the scrubbing action of the pulp on the coatings can remove protective scale and oxides that develop on the coating surface. This action leads to an increased wastage rate and expensive problems in maintenance and equipment down time.
Attempts at mitigating these problems have ranged from the utilization of costly bulk solid materials such as Duplex Stainless Steels, high Nickel alloys and other materials such as Titanium, to the use of coatings to provide protection to standard steel digester construction. The harsh chemical environment, temperature and erosive effect in the pulping digester make the selection of a coating material difficult. Additionally, an application technique that is viable and cost effective for in situ use is required.
Advances in thermal spray processes that produce adherent, dense coatings combined with new alloy materials that have excellent corrosion and abrasion / erosion properties have provided maintenance engineers with a new tool to solve old problems.
Traditionally, the use of thermal spray as plasma, electric arc, and combustion type processes have had only marginal to moderate success. Some of the corrosion problems have been inherently limited by available materials. Many of the problems associated with thermal spray systems is the inability to develop a coating dense enough to provide a true barrier layer. In most coatings the interconnected porosity, oxide stringers and the lack of metallurgical bonding between particles have allowed corrosion pathways to develop to the substrate with resultant corrosion and lifting/Spalling of coatings.
This work presents a summary of a series of tests that were undertaken in order to evaluate the potential of using HVOF applied coatings to protect digesters from corrosion. A number of materials in these tests, have been traditionally used in the industry and some have been recently developed.
