Razumijevanje korozije u rotirajućim strojevima
Korozija je postupno propadanje metalnih površina elektrokemijskim ili kemijskim reakcijama s okolinom, što rezultira gubitkom materijala, hrapavošću površine, korozija, and the weakening of mechanical components. In rotating machinery it attacks shafts, bearings, gears, casings and structural elements, creating stress concentrations that can initiate umor pukotine, hrapave površine koje ubrzavaju nositi, and — in severe cases — causing direct structural failure through loss of load-bearing material. It is often treated as a slow, long-term degradation mechanism, yet it can sharply accelerate mechanical failure, which is why it must be controlled through deliberate material selection, protective coatings, environmental control and corrosion-inhibited lubricants.
1. Definition: What is Corrosion?
At its core, corrosion is the return of a refined metal to a lower-energy, more stable compound — usually an oxide, hydroxide or salt. Most industrial corrosion is electrochemical: it requires an anode (where metal dissolves), a cathode (where a reduction reaction occurs), a metallic path between them, and an electrolyte such as moisture, condensate or process fluid. Remove any one of these and the reaction stops, which is the principle behind almost every prevention strategy below.
Corrosion rarely acts alone. In rotating equipment it usually combines with mechanical loading, so the practical danger is not just lost wall thickness but the way corrosion seeds and feeds other failure modes — fatigue cracking, abrasive nositi, loss of fit, and lubricant breakdown. A shaft that loses a few tenths of a millimetre to general rust may be unaffected, but the same shaft with a single sharp corrosion pit at a keyway can fail catastrophically.
2. Types of Corrosion in Machinery
Uniform (General) Corrosion
- Izgled: Even surface attack across the whole exposed area.
- Primjer: Rusting of unprotected carbon-steel surfaces.
- Stopa: Predictable, quantified as material loss per year (mils per year, or mm/year).
- Učinak: Gradual reduction in wall thickness and a general increase in surface roughness.
- Rizik: Least dangerous form, because the progression is visible and predictable and can be designed for with a corrosion allowance.
Pitting Corrosion
- Izgled: Localised attack creating small cavities or pits.
- Mehanizam: Breakdown of the protective passive film at specific points, where a tiny anode drives deep, concentrated metal loss.
- Opasnost: Each pit acts as a stress concentration that can initiate a umor crack — far more harmful than its small lost volume suggests.
- Common on: Stainless steels and aluminium in chloride-bearing environments.
- Otkrivanje: Visual inspection and eddy-current testing.
Crevice Corrosion
- Mjesto: In gaps, under gaskets and in threaded connections.
- Mehanizam: Stagnant solution trapped in a crevice becomes oxygen-depleted and chemically aggressive.
- Hidden nature: Often invisible without disassembly.
- Common at: Flanges, under O-rings, and at thread roots.
Galvanic Corrosion
- Uzrok: Two dissimilar metals in electrical contact with an electrolyte present.
- Primjer: A steel shaft running in a bronze bearing with water contamination.
- Učinak: The more anodic (electrochemically active) metal corrodes preferentially while the nobler metal is protected.
- Prevencija: Isolate dissimilar metals electrically, or choose materials that sit close together in the galvanic series.
Stress Corrosion Cracking (SCC)
- Mehanizam: Sustained tensile stress combined with a specific corrosive environment drives crack growth.
- Opasnost: Can cause sudden, brittle-looking failure at stresses well below the material’s yield strength.
- Common combinations: Stainless steel with chlorides; brass with ammonia.
- Prevencija: Material selection, stress relief, and environmental control.
Fretting Corrosion
- Mehanizam: Micro-motion plus corrosion at press fits or bolted joints, where repeated tiny slip strips and re-oxidises the surface.
- Izgled: Reddish-brown iron-oxide (“cocoa”) or a fine black powder.
- Učinak: Loosens interference fits and damages mating surfaces.
- Common at: Bearing-to-shaft interfaces and shrink fits subjected to vibracija.
3. Effects on Machinery Components
Ležajevi
- Surface pitting initiates fatigue ljuštenje on raceways and rolling elements.
- Corrosion debris becomes a third-body abrasive inside the bearing.
- Corrosion products contaminate the lubricant and degrade the oil film.
- Bearing life can be cut dramatically — reductions of 50–90% are possible.
Osovine
- Corrosion pits act as fatigue-crack initiation sites, the precursor to a napuknuti rotor.
- Section loss reduces effective diameter and strength.
- Surface roughness degrades bearing and seal operation.
- Fretting at press fits loosens mounted components and shifts the rotor’s balance state.
Zupčanici
- Tooth-surface corrosion accelerates contact (pitting) fatigue.
- Increased surface roughness raises noise and meshing losses.
- Corroded flanks hold lubricant poorly, worsening the wear cycle.
- Tooth-root corrosion reduces bending strength — see also nedostaci zupčanika.
Strukturne komponente
- Reduced load-carrying capacity from section loss.
- Stress concentration at corrosion pits.
- Degraded appearance and reduced overall reliability.
- Foundation anchor-bolt corrosion that causes mechanical labavost and softens the support stiffness.
4. Metode detekcije
Vizualni pregled
- Look for rust, discolouration and pitting.
- Check for corrosion products — white, green or red deposits.
- Inspect fasteners for rust or deterioration.
- Watch for weeping at joints, a tell-tale of hidden crevice corrosion.
Analiza vibracija
Corrosion is not a primary generator of low-frequency vibracija, but its mechanical consequences are very visible to a vibration program:
- Corrosion-roughened surfaces raise broadband high-frequency vibration.
- Pits create impact signatures similar to localised mechanical defects.
- Secondary effects matter most: a corrosion-initiated crack produces the characteristic 2× harmonic growth of a cracked shaft, and corroded bearings show classic kvar ležaja frequencies.
Because the symptoms emerge slowly, periodic u trendu of overall levels and bearing-frequency bands is the practical way to catch corrosion-driven damage before it accelerates.
Nerazorna ispitivanja
When corrosion is suspected, nedestruktivno ispitivanje quantifies it directly:
- Ultrasonic testing: measures remaining wall thickness.
- Frekventna struja: detects surface corrosion and pitting via an eddy-current probe.
- Magnetic particle: reveals corrosion-initiated surface cracks.
- Radiografija: shows internal corrosion in inaccessible areas.
Analiza ulja
Oil analysis catches the chemistry before the mechanics fail:
- Water-content detection (Karl Fischer test).
- Corrosive contaminants such as acids and salts.
- Metal particles released by corrosion.
- pH testing to flag acidic, corrosion-promoting conditions.
5. Prevention and Control
Odabir materijala
- Corrosion-resistant alloys: Nehrđajući čelik, bronca, specijalne legure za teške uvjete rada
- Material compatibility: avoid galvanic couples, or isolate the dissimilar metals.
- Grade selection: match the specific alloy to the specific corrosive environment.
Zaštitni premazi
- Boja: barrier protection for structural steel.
- Pozlaćivanje: chrome, nickel or zinc for critical surfaces.
- Galvanising: zinc coating for outdoor or wet applications.
- Specialty coatings: Epoksidni, keramički, termalni sprej za teške uvjete
Podmazivanje
- Use lubricants formulated with rust and corrosion inhibitors.
- Exclude moisture and contaminants from the system.
- Maintain a continuous oil film that protects the surface — see bearing lubrication.
- Change oil on schedule to remove accumulated water and acids.
Kontrola okoliša
- Effective sealing to exclude moisture.
- Dehumidification for enclosed equipment.
- Ventilation to prevent condensation.
- Enclosures for outdoor equipment.
- Temperature control to avoid repeated condensation cycles.
Prakse dizajna
- Avoid crevices where corrosion can hide and concentrate.
- Provide drainage so moisture cannot pool.
- Design for access to clean and inspect.
- Use sacrificial anodes where cathodic protection is appropriate.
6. Corrosion and the Balancing Workflow
Corrosion quietly undermines balance quality. Material lost from one side of a rotor, product build-up over corroded patches, or a balance weight that creeps on a fretted, loosened fit all shift the mass distribution and push up the 1× neravnoteža response. For this reason a rotor that was corroding in service should be re-checked after cleaning or repair rather than assumed good. In the field this is done without disassembly using a portable two-channel analyser such as the Balanset-1A, which measures 1× amplitude and phase in the machine’s own bearings, lets you correct the new heavy spot, and verifies the preostala neravnoteža against the appropriate ISO 21940-11 grade. Pairing that vibration check with NDT wall-thickness measurement gives a complete picture of both the mechanical and the structural health of a corroded rotor.
Corrosion, although primarily a chemical process, carries profound mechanical consequences in rotating machinery. Its role in initiating fatigue cracks, accelerating wear and creating surface defects is what makes prevention — through sound material selection, protective measures and environmental control — essential to long-term reliability and safety.
