Corrosion Protection of Machined Steel Surfaces

Abstract

Protection of smooth, machined surfaces is challenging as the surfaces do not have the roughness required to provide proper adhesion between the substrate and the coating. This gives poor corrosion resistance and lower coating life compared to grit blasted surfaces. The objective with this work is to evaluate and determine turning parameters that may provide better adhesion properties between a lathe turned steel surface and a polymer coating.

Totally 20 steel samples were machined in a screening of different turning parameters. Surface structure of the samples were first investigated visually, before a selection was examined in cross section in an optical microscope. Based on these investigations six variants were chosen for further testing. A number of new samples were prepared with these turning parameters. The samples were coated with two-component epoxy mastic, and cyclic exposed according to ISO 20340 for seven weeks. The results showed that surfaces turned with high feed rate combined with lower depth of cut and small tool nose radius, provided surfaces consisting of high rectangular peaks (average peak-to-valley height, Rz > 350 m) with high peak overhang (> 40 m). These surfaces did not show any delamination after the cyclic exposure test. Surfaces turned with lower feed rate provided less rough surfaces, hence more delamination occurred. The results obtained in this rapport are in accordance with earlier work carried out on the topic.

Different adhesion measurements available today were evaluated and discussed. The review shows that there are no adhesion tests which provides reliable and quantitative adhesion measurements. Pull-off adhesion test and Cross-cut test were carried out and evaluated in this project. The results indicate that cohesion forces are measured rather than the adhesion between the substrate and the coating. Adhesion at the interface between substrate and coating are higher than measured cohesion. Cross-cut test is semi-quantitative, and provides only an indication of the adhesion forces at the interface.