Surface layers applied to low-alloy steel —mainly 51CrV4 or Ck67— to protect springs and washers against moisture, sea salt, gases, or acids, without having to manufacture them entirely from stainless steel.
Surface treatments applied to low-alloy steel parts —mainly 51CrV4 or Ck67— to protect them against degradation from moisture, sea salt, gases, acids, or other corrosive agents.
Their purpose is to extend the service life of the spring when it operates outdoors, in humid environments, or in contact with aggressive substances, without having to manufacture the entire part from stainless steel.
The standard factory treatment on springs made to DIN 2093 / DIN EN 16983 and DIN 6796 is zinc phosphating with oil, which protects the part during transport and indoor storage but is not sufficient for outdoor applications or continuous corrosive environments. Additional coatings are applied in those cases.
In Belleville washers, very high pressures build up over very small areas of the part —especially at the contact edges—, which can degrade the coating over time in dynamic applications. Choosing the right coating must take into account the corrosive agent, the duty (static or dynamic), the operating temperature, and the part diameter.
The most widely used coatings on DIN 2093 / DIN EN 16983 disc springs and DIN 6796 washers, with their key technical parameters. The salt spray test is performed to DIN 50021 (equivalent to ISO 9227); base material 51CrV4.
| Coating | Thickness | Salt spray resistance | Thermal range | H₂ embrittlement | Recommended use |
|---|---|---|---|---|---|
|
Zinc Phosphate + Oil
Factory standard
|
3 – 8 µm |
~40 – 190 h phosphate + oil/wax |
— | No | Transport and indoor storage |
|
Zinc-rich paint
Customer-applied
|
15 – 100 µm |
Variable depends on thickness |
–40 °C / +60 °C humid · +120 °C dry | No | Large springs · low volumes |
|
Mechanical galvanizing (Ball Plating)
The most common
|
≥ 20 µm |
≥ 240 h Zn ≥ 12 µm ≈ 190 h |
–50 °C / +60 °C humid · +280 °C dry | Risk if process is incorrect | Standard outdoor · dynamic |
|
Dacromet 320 (Grado A / B)
Discontinued
|
5 / 8 µm |
> 480 h / > 720 h replaced by Geomet |
–50 °C / +280 °C | No | Discontinued · contains Cr VI |
|
Geomet 321
Zinc-aluminum flake
|
10 µm |
> 720 h ISO 9227 |
–50 °C / +250 °C | No (non-electrolytic process) | Demanding outdoor · chromium-free |
|
Polyamide Coating
Organic · food grade
|
200 / 50 µm (surface / edges) |
Very high waters · salts · oils · solvents |
–55 °C / +100 °C (+140 °C pico) | No | Static · food industry |
|
Nickel Plating (Electroless Ni-P)
Ni-P alloy 10-13 %
|
40 – 50 µm |
> 4 500 h salt spray record |
–250 °C / +180 °C | Risk under dynamic load | Static · high chemical demand |
Salt spray test to DIN 50021 (≡ ISO 9227). Base material 51CrV4. The results are indicative: actual service conditions may change the coating's service life.
From lowest to highest protection, the five most widely used production treatments for disc springs and Belleville washers in standard steel.
Zinc phosphating with oil is the standard corrosion treatment on springs made to DIN 2093 and DIN 6796. The process deposits microcrystalline metal-phosphate structures onto the base steel; the subsequent oil or wax seals the pores and improves resistance.
Oil is the usual additive; wax is limited to parts with an outer diameter ≥ 100 mm due to application-process constraints.
Mechanical galvanizing — mechanical zinc plating — deposits zinc onto the part through the action of glass beads in a rotating drum, without electric current. It avoids the main problems of electrolytic galvanizing.
The key advantage is the absence of hydrogen embrittlement risk: the mechanical process generates no hydrogen in the part, unlike electroplating processes that require a degassing bake.
Patented technology of zinc and aluminum flakes passivated in an inorganic binder, applied in water by dip/spin or spray. In practice it has replaced the former Dacromet 320 as the industry standard for high-demand metal parts.
It is the most widely used zinc flake coating worldwide in automotive and industrial fastening. Reference for the ISO 10683 standard.
Organic coating that forms a pore-free layer from 200 µm. It tolerates waters, saline solutions, greases, oils, solvents, and oxidizing agents; it offers adequate resistance to dilute acids at room temperature. Approved for the food industry.
Typically applied via the Minicoat system: the parts are heated and dipped into plastic powder that bonds through the stored heat. Electrostatic spray for parts > 90 g.
Nickel-phosphorus alloy deposited onto the surface by chemical reaction, without electric current. The 10-13 % phosphorus content produces an amorphous structure that maximizes corrosion resistance and ductility.
Critical limitation under dynamic load: the electrochemical potential difference between Ni and the base steel can cause accelerated galvanic corrosion if the coating cracks or wears in high-pressure zones.
Tell us your use case and our engineering team will advise you on choosing the optimal solution.
When galvanizing is applied electrolytically (not mechanically) on disc springs, the hydrogen generated during the process diffuses into the steel microstructure and drastically reduces its ductility. In high-strength springs under dynamic load, this phenomenon can cause sudden fracture, documented in industrial valve applications with serious consequences for the system.
Mechanical galvanizing (Ball Plating) minimizes the risk because it deposits the zinc through the mechanical action of glass beads, without any electric current. Geomet 321 and electroless nickel plating eliminate it: both are non-electrolytic processes.
In any case the process must be carried out by a qualified operator: a deficient passivation or degassing bake can reintroduce the risk even in theoretically safe processes.
Indicative decision table. For borderline cases —concentrated acids, sour gas, marine NaCl under dynamic load— or when spring failure means high disassembly costs or risk of catastrophic system failure, we recommend evaluating stainless steel or nickel alloys directly instead of a coating over standard steel.
| Situation / Environment | Recommended coating | Note |
|---|---|---|
| Indoor storage or transport | Zinc Phosphate + Oil | Factory standard |
| Outdoor or humid environment · dynamic application | Mechanical galvanizing (Ball Plating) | The most common |
| Outdoor with T > 60 °C humid · or chromium-free requirement | Geomet 321 | Zinc flake · Cr VI-free |
| Regulatory requirement for Cr VI-free (automotive · RoHS · ELV) | Geomet 321 | Complies with the ELV directive |
| Food industry · static outdoor | Polyamide (Minicoat) | Food grade |
| High chemical resistance · static environment (valves, flanges) | Nickel Plating | > 4,500 h salt spray |
| Acids pH < 6.5 · marine NaCl · extreme environment | Stainless steel (1.4310 / 1.4568) | No longer a coating |
Disc springs and Belleville washers with anti-corrosion coatings are key components in industries where exposure to aggressive environments is constant.
Chassis fasteners, drivetrains, and body components exposed to moisture, road salt, and thermal cycling. OEMs require Cr VI-free zinc flake coatings compliant with the ELV directive for all high-strength fasteners.
Onshore and offshore wind towers, structural flanges, and high-strength bolting exposed to permanent weathering. Zinc flake coatings are the industry standard for wind turbine fasteners due to their resistance in accelerated cyclic tests.
Springs in high-pressure valves, actuators, and flanges in environments with H₂S, CO₂, and seawater. In the presence of sour gas, zinc coatings are not sufficient; stainless materials or nickel alloys (Inconel) are required.
Springs in pumps, reactors, and valves in contact with solvents, dilute acids, or oxidizing agents. Polyamide and nickel plating are the usual options in these environments.
Production lines with frequent washdowns using hot water, steam, and alkaline cleaning agents. Polyamide, approved for food contact, is the first choice in this sector.
Expansion joints, vibration dampers, and structural connections in permanent outdoor use, where surface treatments superior to standard phosphating are required.
Both are zinc-based coatings for springs in standard steel (51CrV4), but they differ in process, performance, and environmental regulation. Mechanical galvanizing (Ball Plating) applies zinc by a mechanical route, offers >240 h in salt spray, and works up to 60 °C in humid conditions. Geomet 321 applies zinc-aluminum flakes by a non-electrolytic route, offers >720 h in salt spray, works up to 250–300 °C, and is free of hexavalent and trivalent chromium, fully complying with RoHS and the automotive ELV directives. For dynamic outdoor applications or where regulations require the absence of Cr VI, Geomet 321 is the first choice.
It depends on the type. Mechanical galvanizing and Geomet 321 are suitable for dynamic applications: the non-electrolytic process eliminates or minimizes the risk of hydrogen embrittlement. Nickel plating and polyamide, on the other hand, are not recommended under dynamic load: nickel plating can cause accelerated galvanic corrosion if the coating cracks; polyamide can wear in high-pressure zones. The technical literature specializing in spring surface treatments specifically warns about the sudden fracture of poorly treated electrogalvanized springs in cyclic applications.
Mechanical galvanizing (chromated Ball Plating) loses its anti-corrosion effectiveness in humid environments above 60 °C. In a dry atmosphere it can work up to 280 °C. If the operating temperature exceeds 60 °C in the presence of moisture, Geomet 321 (up to 250–300 °C) is the direct alternative. For temperatures above 300 °C or highly corrosive environments, stainless steels or nickel alloys must be evaluated.
Stainless steel is recommended when: (1) the corrosive agent is an acid with pH < 6.5, since zinc coatings react with acids and dissolve; (2) the exposure is to concentrated NaCl (seawater) under dynamic load; (3) spring failure can cause costly downtime or safety risks; or (4) it is an application in citric or organic acid, where the resistance of stainless steel clearly exceeds that of any zinc coating. In extreme environments (sour gas, H₂S), nickel alloys (Inconel) are required instead of conventional stainless steel.
For two reasons. First, the high electrochemical potential difference between nickel and the base steel can cause accelerated galvanic corrosion if the coating cracks or wears in the high-pressure zones of the Belleville washer (contact edges). Second, any reduction in ductility in springs subjected to dynamic cycling can cause sudden fracture. That is why nickel plating is reserved for static applications such as DIN 6796 with a demand ≤ 30 % of travel.
The standard test is DIN 50021 (equivalent to ISO 9227 — Neutral Salt Spray, NSS). The values shown in the technical table (240 h, 720 h, 4,500 h, etc.) correspond to this test, on base material 51CrV4. They are indicative results: actual service conditions — temperature, relative humidity, wet/dry cycles, presence of chlorides — can significantly change the real service life of the coating.
Tell us your use case and our engineering team will advise you on choosing the optimal solution.
