Disc Spring (Spring Washer) Corrosion Protection
Mubea belleville disc springs are capable of enduring rigorous loads at very high stress levels. At such levels, it is very critical to protect the disc springs from chemical or electrochemical corrosion. Please reference (Table 5.5) for a list of corrosion resistant surface coatings. In addition, the table details their respective process and comparative corrosion resistance when exposed to a salt spray test per the requirements of DIN 50 021. The surface coating thickness listed in the table for galvanizing and nickel-plating are illustration of realistic protection values. Some disc spring applications require thicker surface coatings, For example, it is possible to achieve a surface coating thickness up to 50 μm with mechanical galvanizing and chemical nickel-plating. When designing disc springs stacks, lateral surface issues with the coatings are possible where relative movement between the springs may occur. Please contact Mubea directly to discuss these concerns.
Zinc phosphate + oiling/waxing
The standard corrosion protection method for disc springs is zinc phosphate and oiling per DIN 2093 and Mubea’s industry standards. Fine-crystalline configurations of metallic phosphate are left on the base metal during the zinc phosphate process. The application of oil or wax to the disc springs surface enables a more durable corrosion resistant coating. Oil is the most widely used additive, as wax is limited to outside diameters of 100 mm or greater due to its production method. The wax application is apt for disc springs with indoor or properly weatherproofed outdoor applications.
Per customer request, Mubea does offer a manganese phosphate coating for disc springs. The manganese phosphate coatings offer no corrosion protection, but do serve as lubricant. If necessary, minimal corrosion protection can be attained by oiling prior to shipping and storage.
The risk of hydrogen disseminating into the surface of the disc spring is possible when the surface coatings are precipitated from water mixtures. The high strength materials used for disc springs are prime suspects for this scenario. The threat of hydrogen embrittlement (process by which high strength materials exposed to hydrogen becomes brittle and fracture) is possible. However, the hydrogen embrittlement risk can be minimized, via a proper thermal treatment process after galvanizing. In addition, the application of surface coatings by electrolytic galvanizing for disc springs should be avoided due to the risk of hydrogen embrittlement.
Zinc is a more basic metallic chemical element than steel. Zinc shields itself by reacting with the environment to form inert protective coatings. If the zinc coating is locally damaged exposing bare steel, the contiguous areas of zinc coating form a galvanic cell with the bare steel and protect it from corrosion. The thickness of the zinc coating on the disc springs is comparative to the level of corrosion protection. If additional protection is required, a chromate finish can be applied in a wide arrange of colors. An ensuing thermal treatment is essential to banish the hydrogen. Galvanic coatings are typically applied for cosmetic principles.
Mechanical zinc plating
Mechanical zinc plating poses similar levels of corrosion protection as electro-galvanizing, excluding the risk of hydrogen embrittlement. The mechanical zinc plating process is defined by applying zinc powder to the disc spring via a barreling method. Industry standard recommends a subsequent chromate coating be applied to the disc spring. The same range of indoor and outdoor applications can be utilized for both mechanically zinc-plated and electro-galvanized disc springs.
Delta Tone/Delta Seal coating
Delta Tone is an inorganic surface coating, consisting of zinc and aluminum compounds. Bright silver coatings with high corrosion-preventive characteristics are acquired through a baking process. The inorganic coating protects the disc spring cathodically and is electrically conductive, hence eliminating the risk of hydrogen embrittlement. Delta Seal is an organically based coating. A stoutly adhesive, chemical-resistant coating is created after an apt application and baking process. Delta Seal can be applied “with interior lubrication” that provides a lifelong dry lubrication. Delta Tone and Delta Seal are heavy metal free. This high-quality corrosion preventive intermediate is commonly used to shield disc springs against corrosion in outdoor applications.
Dacromet is an inorganic, strongly adhesive surface coating consisting of zinc and aluminum in a chromate compound. Dacromet coating provides a high degree of protection against corrosion for disc springs, a high heat resistance and a low coefficient of friction with no threat of hydrogen embrittlement. It is applied by dip centrifuging in a liquid mixture of zinc chromate (low aluminum content), dried and baked. Single disc springs with an outside diameter greater than 40 mm may stick together during the process. Dacromet-coated disc springs are mainly used for outdoor applications requiring a high level of corrosion protection.
Chemical (Electroless) nickel plating
Chemical nickel-plating (electroless nickel plating) is a very eminent surface coating that is wear-resistant, ornamental and corrosion preventative. Hydrogen embrittlement is possible with the chemical nickel-plating process. The process of chemical nickel-plating of disc springs is geared at generating deposits of nickel (nickel phosphorous alloy) with unique properties of consistency and hardness. Disc springs with chemical nickel plating are mainly used in applications with exposure to high mechanical and chemical stresses.