By: Bilal Said, Chief Engineer, Eaton Corporation

A Brief Overview Of Corrosion-Resistant Piston Rod Coating Technology

FIG. 1: Very long cylinder rod undergoing coating process

he large hydraulic cylinders used for dam gate and A fairly common example is the failure of plasma spray coated piston rods cited by the U.S. Army Corps of Engineers other valve opening and closing applications typically Engineering Construction bulletin on Jan 15, 2009. Unfortu- are expected to last from 30 to 50 years. In recent years, nately, laboratory tests do not predict the complete failure however, an increasing number of hydraulic cylinders modes for these brittle mechanically bonded coatings. When rod coatings are compromised, they expose the rod are failing in the field due to piston rod corrosion. substrate to corrosion attack. Corrosion can occur in a num- T ber of forms such as uniform, pitting, crevice, galvanic, stress, or other types. Piston rod coatings are difficult to repair and replace when they fail. It usually can be done, but in most cases it is an unplanned expense that requires special approval and can take months of coordination and execution. Once approved,

20 Tech Directory 2011 www.fluidpowerjournal.com | www.ifps.org typical times for the actual repair can be anywhere without adjacent cracking and bending from long from three weeks to several months. piston rods. The Coating Certification Process The PTA process imparts more heat to the surface Meeting the Joint Industry Project’s (JIP) “Guide- than laser cladding, which can affect coating hardness, line For Qualification Of Wear And Corrosion Protection Piston Rod Coatings increase iron pickup from the substrate, and cause Surface Materials For Piston Rods” through the joint Electroplated hard chrome coatings can be applied piston rod distortion. Fusion clad coatings generally industry standards 3247 and 3295 is the “gold stan- to the surface of piston rods in various thicknesses. are not as hard as plasma and thermal coatings, but dard” for Highly Corrosion-Resistant Alloys (HCRA). Chrome acts as a wear-resistant coating with good when applied properly with the correct choice of Eaton’s recently completed DNV certification of its lubricity partly because numerous micro-cracks coating material, they provide excellent corrosion Eatonite™ laser cladding as an HCRA provides an in this coating help capture hydraulic oil, which resistance. example of how the process works. improves rod seal lubrication. Those micro-cracks, Eaton started by performing extensive powder however, also are passageways for the atmosphere to coating and application technology testing and analysis gain access to the base material. Other Factors to identify the combination that optimally addressed Plasma spray piston rod coatings are applied with Coated piston rods need to be finished to produce customer requirements. Results were evaluated for a spray gun that impinges the powder on to the sub- a surface roughness profile that will optimize rod and robustness of cladding processes, corrosion resistance, strate. The speed of the deposition process results seal life without compromising the coating’s anti-cor- coating hardness, ductility, process control, and in a porous, lamellar grain structure with particles rosion properties. Surface finish parameters including metallurgical evaluation. being flattened at the site of impact on the piston average maximum profile height, bearing ratio, skew- Most of the pre-evaluation work was done in rod surface. In most cases sealants are used to close ness, and some others need to be carefully specified and conjunction with the DNV laboratories in Høvik, the porosity. controlled to assure optimum performance in the field. Norway, and Columbus, Ohio. Through this process, Plasma spray coatings are generally brittle. Since Application-specific structural integrity consid- Eaton developed its own exclusive powder and they are mechanically bonded, factors such as sub- erations, including the diameter of the rod and the proprietary laser application processes to optimize strate cleanliness, kinetic energy, temperature, pow- material from which it is fabricated, also influence cladding performance. der properties, cooling rates, and others all influence the selection of coating technologies and materials. The DNV certification was divided into three key bond strength. For example, a long-stroke piston rod can bend with phases. In the first phase, DNV reviewed the back- Thermal spray piston rod coatings such HVOF its own weight due to piston rod sag creating micro- ground documentation related to the technology and (High Velocity Oxy-Fuel) use an oxygen fuel mixture cracks in brittle coatings. the manufacturing procedure specifications. As a part that is ignited in a combustion chamber and pro- The duty cycle also must be considered. Some of the certification basis, a critical parameter list was pelled with the powder on to the piston rod surface at applications require constant motion while others developed. Typical documentation included high kinetic and thermal energy. need only a few cycles a day or even less. In hydro- • design basis including description of the technol- The coatings generated are usually denser and power applications, seasonal variations typically play ogy, functional limitations, and main data, reli- less porous and brittle than plasma spray coatings. a role on the extent and frequency of stroke. ability targets and acceptance criteria, reference Although adhesion is better than plasma spray, these Every application has its own set of challenges of applicable codes and authority requirements coatings are still mechanically bonded and typically including static and fatigue generated by shock, exhibit a lamellar grain structure. vibration, side load, eccentric loading, torsion, and Plasma transfer arc (PTA) welding and laser clad- buckling. In addition, factors like rod orientation, ding both create a fusion weld attachment with the length and diameter, temperature, geographic seismic piston rod surface that has excellent adhesion. The requirements, as well as the rules of governing bod- deposited metallic coating is far more ductile than ies such as the Det Norske Veritas (DNV), ABS, DV, plasma or thermal spray coatings. This gives it an Lloyds, and others, all have a influence on the coating ability to absorb impact and handle abrasive scratches choice and performance. FIG. 2: Laser coating of a cylinder rod

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• principles of storage, transportation, installa- of prescribed tests per JIP 3247. Eatonite successfully tion, commissioning, operation, maintenance, completed all three phases and was certified as the and abandonment first, and to date the only, cylinder coating to fulfill • interface requirements the “Guideline For Qualification Of Wear And Corro- • environment sion Protection Surface Materials For Piston Rods.” • functional loads In practical terms, that means the coated cylinder • principles of manufacturing and quality assurance rods deliver the mechanical performance of tradi- • data sheets for chemical pre-treatment products tional SAE 4130 steel piston rods, combined with and masking material where applicable the corrosion resistance expected for materials like • coating applications (including control of essen- Inconel 625, giving customers the best attributes of tial process parameters and instrument settings) both materials. The involvement of international organizations The purpose for this phase was to understand and like DNV working cooperatively with industry lead- review the documentation of the critical parameters ers promises a new generation of HCRA coatings. and overall certification plan, and identify key chal- Corrosion may be an inevitable natural process, but lenges and uncertainties. new materials and technologies like Eatonite are giv- The second phase of the certification process con- ing engineers and end users improved tools for the sisted of on-site audits of the coating procedure dur- fight to control it. ing the manufacturing and finishing process. During the onsite audits, a review of the quality control mea- For more information, visit sures and acceptance criteria for the finished Eatonite clad coating was conducted. The purpose of this phase www.eaton.com FIG. 3: Close-up view of laser was to ensure that the coating tested during the cer- coating of a cylinder rod tification was representative of the metallic cladding. The final phase confirmed that the functional requirements and target reliability/functionality as stated in the guideline can be met through the battery

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