Zinc coated precision steel tubes (galvanized tubes) are destined for hydraulic and pneumatic pressure distribution (so called HPL tubes), production of automobile components, utility vehicles as well as for production of machine products such as fittings and joints. Cold drawn precision tubes are used as the semi-finished product and they are made from rolled seamless tubes. Precision tubes HPL types are made from carbon and alloyed steel.
Length of the galvanized tubesin diameter 4 - 16 mm 6000 + 100 mm
in diameter 18 - 42 mm 6000 + 50 mm
Tolerances of the zinc coated tubes- tolerances of the outside diameter:
|diameter between 4 - 30 mm||±0,08 mm|
|diameter between 35 - 38 mm||±0,15 mm|
|diameter up to 42 mm||±0,20 mm|
- tolerance of the wall thickness ± 10%
The Steel grades for the zinc coated tubesE 235+N and E 355+N
Surface condition of the galvanized tubesFirst layer – electrolitically leached zinc (Zn) – acts as anode and in a corrosive environment it corrodes first and the base metal is cathodically protected against corrosion. The zinc layer thickness may be in the range of 5 to 30 micrometers (µm).
Second layer – protects against the first layer being attacked. The second layer based on the valency of used chrome is called either: passivation or chromating. For chromating in the past the Cr+6 salts were used. Their use has been forbidden based on the decision of EU, it is the ELV Directive – End of Life Vehicles No. 2000/53/EC - Elimination of Used Vehicles and Directive No. 2002/95/EC (so called RoHS Conformity). Nowadays the term chromating is no longer used, but passivation, or based on the number of chromites (salts Cr3+) we are talking about chromiting.
For the overall increase of coating resistance, we use as the last layer the layer of organic substances on the Si base that stabilizes the chrome layer. This process is called impregnation or sealing.
Allowable pressures for hydraulic and pneumatic distributionsHighest allowable pressures for hydraulic and pneumatic distributions in dependence on steel and tube testing are presented in tables and calculation was done according to DIN 2413.
Straightness of the tubes for hydraulic and pneumatic distributions3 mm / m (1 : 1 000), overall maximum 0.0015 % of the overall length of the tubes
Ends of the galvanized tubesEnds of the tubes are smooth and are enclosed by plastic plugs.
Marking of the galvanized tubesMarking of metallic zinc coatings with subsequent modification without Cr(VI) is described in DIN 50979. Because during passivation without the Cr(VI) content we are talking about new systems, thus the new nomenclature was created. Zeleziarne Podbrezova a.s. offers the following:
- thin layer passivation (transparent) is colorless perhaps slightly rainbow color. It is marked as A,
- strong layer passivation (rainbow – iris) it has a rainbow tint. It is marked as C,
- surface without sealing - there is no code. A sealed surface has T2 code.
Example of surface marking Fe / Zn8 / A (it means – the base material is steel, layer of zinc 8 micrometers, transparent passivation, without sealing)
Testing of the galvanized tubesThe electromagnetic nondestructive testing of leak tightness is always performed according to EN 10246-1 or other corresponding standards. Selection of the tests is defined in EN 10305-1. On the top of it the NDT test and the flattening or drift expanding test is possible.
Testing of resistance of galvanized tubes surface
Type of protection
Minimum Corrosion Resistance Salt Spray (Fog) Test [hour]
Cr VI Free
|Blue Passivating (bright)|
|HRI Silver Passivating (High Resistance Iridescent)|
According to the following standards: EN 10 204 or DIN 50049
- nonspecified testing - 2.2
- specified testing 3.1.B or 3.1
- upon agreement - 3.1.C or 3.2