What steel the adapters and fittings are made of, what they are plated with, what the salt-spray hours mean and how to read a test report. Everything that usually gets asked before a first order.

Adapter bodies, studs, nuts and ferrules in the catalogue are made of carbon structural steel. That is a deliberate choice rather than a saving: carbon steel machines and forms well, holds hydraulic working pressure with a wide margin and costs a fraction of stainless. In a hydraulic circuit the part runs in oil on the inside and is protected by plating on the outside, so corrosion resistance of the steel itself is seldom needed.

The grade is dictated by how the blank is produced. Simple shapes — ferrules, nuts, short studs — are best cold-headed from bar or wire: the metal is not cut away but displaced, the grain follows the contour of the part, and strength is higher than in a part turned from the same bar. That calls for a ductile, low-carbon steel. Elbow, tee and adapter bodies of complex shape are turned from hot-rolled bar or die-forged and then machined; there a steel with more carbon is acceptable, and it holds threads and seat cones better.

After cold heading, ferrules are heat-treated: the work-hardened metal is annealed to restore ductility, because a crimp ferrule has to flow under the dies of the press, not crack. That is one reason ferrules and fittings come from different works — different blanks, different machines, different regimes.

• Carbon steel is the standard for every series in the catalogue unless the series description says otherwise.
• Stainless steel AISI 304 or 316 is possible to order for some series; ask about lead time and minimum quantity when you enquire.
• Brass is not used in high-pressure hydraulic fittings — only in pneumatics and low-pressure lines.

Which parts are forged, which are turned from bar and which are cold-headed from wire is explained in the article “Manufacturing process”.

On the outside the part is protected by electroplated zinc. Zinc does not work like paint but as a sacrificial layer: it is electrochemically more active than steel and corrodes first, covering the steel even where the coating is scratched. As long as there is zinc left on the part, there will be no red rust on it.

Coating thickness on hydraulic fittings is usually 5 to 12 microns. It is not made thicker: threads and seat cones are toleranced to hundredths of a millimetre, and extra zinc on them interferes with assembly. That is why you cannot ask for “a thicker coating” on a fitting the way you can on a bolt — resistance is gained through passivation, not thickness.

Passivation is a thin conversion film on top of the zinc that slows down the zinc’s own corrosion, that is, it delays the appearance of white deposits. Until the mid-2000s this film was based on hexavalent chromium. It is now banned in Europe under RoHS and ELV, and the whole catalogue is passivated with trivalent chromium, Cr(III). Modern trivalent films with a sealer match the old ones in resistance, but they must be named in the specification: “zinc with Cr(III) passivation”, not just “zinc plated”.

The colour of the coating tells the type of passivation. Bluish silver is the thin film, the most common option on fittings. Iridescent, with a yellow-green sheen, is thick-film passivation, and it gives more hours in salt spray. Black is found on decorative parts and is rare in hydraulics. Colour does not affect threads or interchangeability, only resistance and appearance.

Coating resistance is tested in a salt-spray chamber to ISO 9227 (ASTM B117 in American specifications). The part hangs in the chamber at 35 °C while a five per cent salt solution settles on it continuously. Hours are counted to two events.

Reference figures for 8 microns of electroplated zinc: blue Cr(III) passivation — 72–120 hours to white corrosion; thick-film iridescent — 120–200 hours; with an additional sealer — 200–300 hours and more. The baseline requirement for the catalogue coating is at least 96 hours to white corrosion; the batch test report is available on request.

• Salt-spray hours cannot be converted into years of service: it is a comparative test, not a forecast. It lets you compare two coatings with each other, not say how long a part will last on a particular excavator.
• The test is run on flat panels or on the parts themselves; on a thread the result is always worse than on a smooth surface — that is normal and should be stated in the method.
• If a specification says “240 hours” without qualification, ask: to white or to red corrosion. The difference between the two requirements is a factor of three to four in the cost of the coating.

1. Test method: ISO 9227 NSS (neutral salt spray) or ASTM B117. Other regimes — AASS, CASS — are harsher and are not used for zinc.
2. What was tested: witness panels plated together with the batch, or parts from the batch. The second is the more representative option.
3. Coating thickness measured before the test, usually by X-ray fluorescence to ISO 3497, on a flat area.
4. The criterion: hours to the first traces of white corrosion and to the first traces of red; sometimes the affected area in per cent to ISO 10289.
5. Type of passivation and whether a sealer was applied. Without that line the hours cannot be compared with anything.

O-rings in ORFS fittings, in metric face-seal connections and in SAE O-ring boss adapters are made of NBR, nitrile rubber. It is the standard material for mineral hydraulic oils: working temperature from −30 to +100 °C, briefly up to +120 °C. For high temperatures, ester-based biodegradable fluids and aggressive media the rings are replaced with FKM (fluoroelastomer, trade name Viton): −20 to +200 °C. Rings are supplied fitted; a change to FKM is possible on request for a batch.

The bonded seal under BSP is a steel washer with a rubber ring bonded inside. The washer is zinc-plated like the fitting, the rubber is NBR. The seal is single-use: it is replaced after disassembly.

What harms zinc plating is not salt but condensation: in a closed container, when the temperature swings, dew forms on cold steel and white deposit appears in weeks rather than years. So parts are packed in polyethylene with a corrosion inhibitor (VCI) and cartons go on pallets in stretch film. In the warehouse keep relative humidity below 60 % and no direct contact with stainless-steel or copper parts: paired with them, zinc corrodes faster.

• Do not remove the factory packaging before assembly if the parts are going into long-term storage.
• Threaded port plugs are not packaging but protection for the seat cones; remove them right before assembly.

If you have a coating requirement — from a tender, from an end customer or from your own catalogue — send it as it is. Four lines are usually enough: material, zinc thickness, type of passivation, hours to white and to red corrosion. Against those we either confirm the standard finish or propose the nearest option; the batch test report is available on request.