Zistaplex Fachartikel - Rohtext_en - VOSS

Translation from O+P Magazine 08/2009
Drastically-improved
characteristics
Optimum corrosion protection in hydraulic connection technology
Harald Pott, Helwig Brabander
The changeover to chromium-VI
VI-free galvanised surface protection coatings resulting from
the EU End-Of
Of-Life Vehicle Directive ive occurred a few years ago. Manufacturers of hydraulic
connection elements have developed and marketed a variety of solutions since then. In
addition to familiar galvanised connection elements, solutions with drastically-improved
improved
characteristics are also now available on the market.
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The use of zinc-nickel in particular as a basis for the corrosion protection coating has established
itself on a broad front for hydraulic connection elements. However, chromium-VI-free surface
protection coatings for hydraulic precision steel tubes are not only available today from connection
element manufacturers, but also from tube manufacturers themselves.
Corrosion protection for tubes and tube connections
Not everything was better in the good old days, but at least things were more uniform. When it came
to corrosion protection, galvanising of steel components had established itself as the alternative to
the even older phosphating technique, with the zinc coating being then subjected to chromating. The
coating structure was, and still is, known by the designation A3C and was employed for yellow
chromated components such as screw fittings, valve blocks or tubes. However, the chromium-VI
prohibition ended this uniform situation, and the vanishing of chromium VI ions simultaneously led to
the disappearance of the positive corrosion protection properties of the A3C coating. Chromium-VIfree
passivation introduced as an alternative was initially far from capable of achieving the service
lives usually determined previous to this in standardised tests. Sealing layers were utilised as
supplementary protection to avoid this. However, VOSS Fluid went a step further to achieve a true
improvement and avoid any setbacks from a technical point of view. Instead of utilising the old basic
zinc coating further and continuing to rely exclusively on the improved properties of passivation and
sealing, a zinc-nickel coating is used for VOSS screw fittings. When compared to pure zinc, the
corrosion resistance of zinc-nickel is now increased several fold in the basic coating, thus improving
the corrosion resistance of the screw fitting. The remaining coating structure is similar to that of
galvanised screw fittings. Chromium-VI-free passivation follows the zinc-nickel coating which, in turn,
is subsequently subjected to sealing. In contrast to other chromium-VI-free protective coatings, the
sealing technique employed by VOSS Fluid is an anorganic version. In addition to being media
compatible to a limited degree, this also ensures in principle the compatibility with all fluids generally
used in hydraulics. Of decisive importance in deciding to use zinc-nickel for corrosion protection
was, primarily, the life history of a hydraulic screw fitting. The surface must meet special requirements
during handling and fitting. Galvanic coating of individual parts follows machining under practical
conditions. These are then stored for a certain period of time on the manufacturer’s premises prior to
being utilised further for subsequent installation operations. They are then either supplied directly to
the customer or subjected to renewed storage (e.g. by retailers). Damage to the protective coating
can already occur during transportation when fitting and handling the parts. However, damage to the
coating can at the latest no longer be avoided during assembly of the screw fittings in hydraulic
systems. The excellent and, indeed, desired corrosion protection properties of VOSS zinc-nickel
surfaces were confirmed through inspections conducted by a reputable independent institute.
Service lives of at least 720 hours were achieved during standardised salt spray testing before the
first occurrence of red rust was detected. The service life of the zinc-nickel basic coating is
(calculated using a rule of thumb) around factor 10 higher than a conventional zinc coating. VOSS
Fluid has been utilising this type of coating as a standard coating for all product series for more than
three years. Experience gained during this period corroborates the laboratory trial results. The
excellent corrosion protection properties if zinc-nickel are in particular clearly demonstrated in
applications in the area of mobile hydraulics.
VDMA Standard Sheet 24576
A standardised designation for chromium-VI-free surface protection coatings of hydraulic
components was until recently still missing. A working group was formed in the VDMA to close this
gap and address the subject of chromium-VI-free corrosion protection coatings for fluid engineering.
VDMA Standard Sheet 24576 is the result of the efforts invested by this group over a period of one
and a half years. This standard sheet provides assistance in the labelling of chromium-VI-free
corrosion protection coatings for fluid engineering components with a short abbreviation which is
easy to remember. The revolutionary aspect of this new standard sheet is that the structure of the
coating is no longer regarded as the central point of emphasis of the designation, but rather the
effectiveness. Corrosion protection classes were introduced to define this effectiveness. These
corrosions protection classes (K1 to K5) set differing requirements for the effectiveness of protective
coatings. A minimum requirement of 720 hours must be ensured in the highest class before red rust
is detected. An English-language version is currently being compiled. In addition to the corrosion
protection class, this standard sheet also offers an option for the identification of features such as the
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coating structure and surface coating colour for those who would prefer greater precision. The VOSS
zinc-nickel coating can thus be completely labelled with the abbreviation K5S6F2, representing the
highest of all corrosion protection classes with a silver-grey appearance.
ZISTAPLEX – outstanding corrosion protection for tubes
Benteler Stahl/Rohr has been manufacturing chromium-VI-free galvanised precision steel tubes for
years under the name ZISTA CRF. The tubes are utilised in applications such as vehicle tubing and
for machine and plant construction. However, the coating must meet higher requirements in
numerous applications. The combination of surface damage (e.g. in the case of tube screw fittings
during transportation, handling and further processing) and the absence of the self-repairing effect of
the chromium-VI-free coating leads to a limitation of corrosion protection characteristics. Benteler
Stahl/Rohr offers welded and seamless ZISTAPLEX tubes in the chromium-VI-free product range as a
solution to this problem. BENTELER ZISTAPLEX considerably improves the corrosion resistance of
galvanised tubes through an additional organic coating. A classification of ZISTA CRF in corrosion
protection class K3 in compliance with the VDMA standard sheet places BENTELER ZISTAPLEX in
the highest class K5. The complete designation for a black surface (to give an example) is K5F3SX.
In addition to providing outstanding corrosion protection, BENTELER ZISTAPLEX is also
distinguished by excellent plasticity which meets all requirements, even after complete machining of
the tubes. This dispenses with frequently complex and costly galvanising and painting on the part of
the customer. The special production process enables the flexible use of different organic coating
types. In addition to free colour selection, special requirements such as temperature and chemical
resistance can thus be met. A solvent-free process is employed for environmental and personnel
safety reasons, simultaneously fulfilling the requirements of the EU Solvents Directive. The process
also ensures a uniform coating distribution which, in turn, ensures uniform corrosion protection over
the entire tube and enables use of the most common DIN and SAE tube screw fittings without prior
removal of the coating.
Interplay of components
The availability of ZISTAPLEX tubes from Benteler Stahl/Rohr raises the question of the degree to
which the tube screw fittings/tube system harmonises with each other. The objective of a joint project
realised by Benteler Stahl/Rohr and VOSS was therefore to conduct suitable tests and inspections to
examine the results which could be achieved with this combination. The test program was laid down
mutually, encompassing corrosion protection tests on the one hand and static and dynamic strength
testing of the connections on the other. ZISTAPLEX tubes from Benteler Stahl/Rohr conforming to EN
10305-4 with the dimensions 12 x 1.5 were utilised as test samples. The 2S plus seal cutting ring from
VOSS and the VOSSForm SQR tube connection system were defined as typical tube connection
systems.
Test program
Pre-assembly of the seal cutting rings on the hydraulic tube was realised with the VOSS Type 90
Basic pre-assembly device in compliance with 2S plus seal cutting ring assembly instructions. Tube
connection cutting assemblies were subjected to random examination through grinding to ensure the
correct assembly of all test samples. The pre-assembled tubes then underwent leak and burst
pressure testing and a pressure pulse test (1 million load changes at 1.33-fold the nominal pressure)
to check dynamic strength. In addition to these standard tests, a burst pressure test was conducted
on the assembled tubes, these being previously subjected to an alternating temperature load. The
alternating temperature test was conducted to determine the influence of the 50 µm thick outer
plastic coating. The tubes were cooled to -20 °C for this purpose, maintained at this temperature for 3
hours, then heated to + 80 °C and likewise maintained at this temperature level for 3 hours. 10
repetitions of the cycle were realised. All pressure tests were conducted relative to a nominal tube
pressure of 300 bar. An identical test program followed with VOSSForm system formed tubes. The
test program for this connection system also involved salt spray testing of the assembled tubes.
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Test t results
2S plus
A slightly raised coating forms above the material flare in front of the abutting surface of the seal
cutting ring. Loose coating residue is removed prior to installation to ensure flawless functioning of
the hydraulic system. This can be realised without difficulty through wiping. The leak, burst pressure
and pressure pulse tests conducted with the test tubes prepared in this manner were all passed
successfully.
VOSSForm
As is the case with the seal cutting ring connection described previously, coating residue must be
removed through wiping. This ensures correct assembly and the reliability of repeat assemblies. All
the assembled VOSSForm tubes also successfully passed the leak, burst pressure and pressure
pulse tests conducted subsequently. Tubes subjected to prior alternating temperature load also
exhibit no faults. The results of the salt spray test are of interest in the case of VOSSForm formed
tubes. These also indicate no basic metal corrosion after 720 hours in the salt spray chamber.
Results from practical applications
In cooperation with Goldhofer, the system tested together by VOSS and Benteler Stahl/Rohr was
utilised for the pipework on a heavy goods vehicle. Goldhofer, a company located in Memmingen, is
one of the leading manufacturers of special transportation vehicles on an international level. Around
1,050 employees produce vehicles with payloads ranging from 25 to 10,000 t at the approximately
120,000 m² company premises. More than 32,000 vehicles have been manufactured to date in over
70 countries around the world. In order to improve internal procedures and enhance the quality of
their products, Goldhofer joined with VOSS and Benteler Stahl/Rohr to test the use of the VOSSForm
system in combination with BENTELER ZISTAPLEX tubes. The experience gained here in the area of
assembly was uniformly positive. The heavy goods vehicle is currently being subjected to field tests
by the customer. Use of zinc-nickel in conjunction with ZISTAPLEX enabled Goldhofer to improve
internal production/ assembly processes considerably. The outstanding corrosion protection of
components enabled Goldhofer to dispense with the subsequent painting of hydraulic units, an often
complex and costly procedure. In particular, those parts which are difficult to reach in the paint shop
are considerably better protected against ambient corrosive attacks. “The choice of VOSS zinc-nickel
and ZISTAPLEX from Benteler Stahl/Rohr enables us to improve the excellent quality of our products
even further”, says Mr. Unglert, a senior hydraulics fitter at Goldhofer.
Summary
Test results confirm the perfect interplay between VOSS zinc-nickel and BENTELER ZISTAPLEX.
Corrosion resistance in the highest class K5 as defined in the VDMA standard sheet can be achieved
without difficulty. The coating of the tubes in no way impairs the function of the VOSS 2S plus seal
cutting ring and VOSSForm tube forming system. Goldhofer, which sets stringent leak-free and
corrosion-resistant connection criteria, confirmed the effectiveness of product pairings under real
practical conditions.
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1: Complete VOSS zinc-nickel nickel surface coating structure
2: Damage caused to the corrosion protection coating during the assembly procedure
3: Schematic depiction of the BENTELER ZISTAPLEX tube surface coating structure
4: VOSSForm
test samples from the salt spray test
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5: Assembled 2S plus seal cutting ring on BENTELER ZISTAPLEX tube
7: Formed VOSSForm
tube end of BENTELER ELER ZISTAPLEX tube
6: Correct cut of seal cutting ring assembly into tube
8: Subsequent painting of components is dispensed with
Authors: Dr. H. Pott, Area Manager for Development and Design, VOSS Fluid GmbH, Wipperfürth, H. Brabander (Graduate Engineer),
Head of Surface Technology, Benteler Stahl/Rohr GmbH, Paderborn
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