How Corrosion Affects Pump Performance — And What Genuine Parts Do Differently

Corrosion in pump internals is not just a surface degradation problem — it is a hydraulic performance problem, a sealing integrity problem, and a structural reliability problem simultaneously. The pump components most sensitive to corrosion are also the most critical to performance: impeller vanes, wear rings, casing surfaces, shaft sleeves, and seal faces. Understanding the mechanisms and the material decisions behind genuine parts explains why corrosion behaviour differs so significantly between OEM and generic replacement components.

Corrosion Mechanisms in Pump Components

General (Uniform) Corrosion

General corrosion attacks the metal surface uniformly across all exposed areas. In cast iron pump casings and impellers, general corrosion occurs in contact with water containing dissolved oxygen, particularly in applications with significant temperature cycling that disrupts the protective oxide layer. The hydraulic consequence is a progressive increase in surface roughness — which increases friction losses in the flow passages, reduces efficiency, and eventually causes dimensional changes that affect clearances.

Galvanic Corrosion

Galvanic corrosion occurs when two dissimilar metals are in electrical contact in a conductive fluid. In mixed-material pump assemblies — cast iron casing with stainless steel impeller, for example — the less noble metal (typically the iron or carbon steel component) corrodes preferentially. Correct material selection for the pump application, and the use of appropriate isolation between dissimilar materials where required, prevents this mechanism.

Crevice Corrosion

Crevice corrosion occurs in geometrically confined spaces where the fluid is oxygen-depleted — O-ring grooves, bolted flanges, threaded connections. The locally low oxygen content creates an electrochemical concentration cell that drives accelerated corrosion at the crevice. Stainless steel components, despite their general corrosion resistance, are particularly susceptible to crevice corrosion in chloride-containing media — a relevant concern for pumps handling seawater or brine.

Erosion-Corrosion

Where fluid velocity is high — particularly at impeller vane tips, volute cutwater, and wear ring surfaces — the combination of fluid momentum and electrochemical corrosion produces erosion-corrosion. The flowing fluid continuously removes the protective oxide layer that would otherwise limit corrosion rate, exposing fresh metal to further attack. Applications with abrasive particles in the pumped fluid accelerate this mechanism dramatically.

How Corrosion Changes Hydraulic Performance

The hydraulic consequences of corrosion follow from the dimensional changes it produces:

• Impeller vane roughness — increased surface roughness on impeller vanes raises friction losses in the flow passages, reducing head and efficiency. A moderately corroded impeller can show 5–8% efficiency reduction compared to its new condition
• Wear ring clearance growth — corrosion at the wear ring interface enlarges the clearance from both sides simultaneously — the rotating impeller hub and the stationary casing ring. The increase in internal recirculation further reduces performance
• Volute surface degradation — roughening of the volute surfaces increases hydraulic losses in the pressure recovery section, reducing overall pump efficiency

Material Choices in Genuine vs Generic Parts

The alloy specification for genuine Flygt and Lowara parts is chosen for the corrosion environment of each specific application:

• Standard water applications use grades of cast iron or ductile iron with controlled composition for predictable corrosion behaviour
• Aggressive or chemical applications use stainless steel grades selected for the specific corrosive medium — not generic 304 or 316 stainless, but the grade with the correct nickel, molybdenum, and chromium content for the application
• Wear ring materials are selected not only for corrosion resistance but for the correct wear couple with the opposing material — the hardness differential between rotating and stationary rings must be maintained to prevent seizing

Generic replacement parts use commercial alloy grades that meet standard specifications for the named material — but not necessarily the specific composition range that the OEM has validated for the application. The difference may not be visible on inspection. It becomes visible through accelerated corrosion and reduced service life in conditions that the generic alloy is not optimised for.