Wear-resistant oxide coatings for aluminium impellers of rotary gas meters: a clearance-based assessment of hard anodizing and plasma electrolytic oxidation

Rotary (Roots-type) gas meters measure volume by displacement between two figure-eight impellers that rotate without contact, separated from each other and from the housing by clearances of a few tens of micrometres. Abrasive particles carried by natural gas, in particular the iron oxides, oxyhydroxides and sulfides of black powder, widen these clearances, increase the slip flow and shift the error curve negatively, most strongly at the minimum flow rate. This preprint analyses how the wear resistance of aluminium-alloy impellers can be raised by oxide coatings without degrading metrological performance. A slip-flow model is used in which the gap conductance grows with the cube of the clearance and the pressure difference at low flow is set by mechanical friction; it yields a closed-form expression for the permissible clearance growth of a meter that must stay within its maximum permissible error. For nominal clearances of 25–100 µm, the permissible wear is about 1–30 µm per surface, in most cases less than the thickness of an oxide coating, so the service interval is governed by the wear rate of the coating rather than by its thickness. Published data are then compared using the ratio of particle to surface hardness. Hard anodic coatings (≥ 400 HV) remain in the hard-abrasion regime against the iron oxides and oxyhydroxides, pyrite and quartz, whereas the dense α-Al2O3-rich layer of coatings produced by plasma electrolytic oxidation (PEO, assumed 1200–1800 HV) moves most of these particles into the soft-abrasion regime; a published dry-sand abrasion study on 6061-T6 alloy showed a twofold and a 12–30-fold reduction of wear, respectively. Design rules are derived for the growth allowance, for sealing and masking, and for the edge radii, whose leakage penalty scales with the fourth power of the radius and, for hard anodic coatings, of the coating thickness; a manufacturing route and a test programme are also proposed.