Choosing Your Anti-Seize
An anti-seize compound is by nature a dissimilar material sandwiched between 2 like metal alloys to reduce the potential for galling during the assembly of bolted joints whether it be traditional flanges or valve gland flanges. When considering anti-seize/thread lubricant formulations, the lubrication is made up of two stages. The use of a high-quality grease serving as the base lubricant up to 400° F, with additional solids for lubrication suspended in the grease to allow for use in temperatures above 2,000° F. It is important to understand that as temperatures exceed 400° F, the grease will dissipate, leaving and the lubricating solids to plate the surfaces after they have softened.
Anti-seize/thread lubricant materials may perform differently depending on the stud material that they are applied to, their operating temperature and environment. There are many bolting materials typically used in the Hydrocarbon Processing Industry (HPI) and the Chemical processing industry (CPI). ASTM A193 B7 studs, which is the most common used and are carbon steel by composition. Another commonly used stud material is B16, which is like B7 studs but higher in molybdenum and contains vanadium. ASTM B8 studs is a different grade of stainless or Austenitic alloy type that are sometimes used as well. Because of the variability of stud materials, there has been many different anti-seize/thread sealant products developed over the years. We have identified some of the more common materials used to manufacture ant-seize/thread lubricants.
Graphite comes in many different types and can even come as oxidation inhibited grade. Typically, graphite is listed as good up to 2820 F, but that would likely be in a reducing or vacuum environment. In furnace tests, graphite will totally disappear within 4 hours at 1500F. It takes longer at lower temperatures but indicates the carbon will react with oxygen present to convert to carbon dioxide. One would not expect the graphite, when sandwiched between the thread flanks and nut faces, to completely convert to carbon dioxide but the helical void between the thread roots and crests and other areas will be affected. Since graphite has only 1.0 Mohs hardness, it might not be adequate to separate the surfaces under high contact pressures, such as the 80,000 psi one can attain in engagement of fasteners when loading larger flanged joints. Another issue could be a reaction with the steel itself such as carburization. Another significant issue could be graphite’s contribution to galvanic corrosion. On the galvanic series, graphite is more noble than gold. This means it will most likely result in corrosive pitting of the component/fastener assembly it is intended to protect in certain alkaline environments where low levels of electrical current can occur. This potential can be reduced by the addition of other chemical or solid additives to the anti-seize composition.
Graphite quality and properties can also have a significant impact on anti-seize performance. Graphite materials with low oxidation temperature or high ash content can significantly increase the torque needed to disassemble the flanged joint.
Nickel is not a dissimilar metal to many of the alloys used as fasteners. Although the classic temperatures for reactions or dissipation of one material surface into another may be much higher, with prolonged time, pressure and temperatures, reactions will progress.
Copper (or Brass/bronze) is often a component in anti-seize compositions. It is ductile making it a good plain bearing material. Copper and its alloys are also dissimilar materials for most fastener alloys. Therefore, it tends to make an excellent barrier against galling. Formulas made using copper are limited to 1,800°Fand can handle most service requirements. However, copper is not typically used in refineries due to the poisoning of catalyst beds. Additionally, copper is reactive with materials such as acetylene, vinyl monomers, certain catalysts, etc. found in many chemical processing applications. Copper is highly conductive, which can be good or bad depending on the application, and since it is higher on the galvanic series of elements, is not recommended for aluminum fastener applications.
Molybdenum disulfide is an excellent friction reducer and anti-galling additive. It can easily burnish to a surface and provide both a chemical reaction (to form iron or other metal sulfides which are softer than the steel bolting components to reduce galling and friction) while also placing a dissimilar solid between the two similar composition sliding surfaces of the fastener set. Using molybdenum disulfide results in significant reductions in coefficients of friction at temperatures significantly under 400F, adding a need for caution where hot torqueing may be required. (“Hot torqueing” means re-torqueing at elevated temperature back to the original targeted stud stress to make up for gasket relaxation.) There may also be concerns over time and temperature of sulfide stress crack potential and potential for galvanic corrosion. However, additives can be formulated into the molybdenum disulfide products to reduce or eliminate these concerns.
Zinc is another common anti-seize material used in Mil Spec products such as Zinc Dust-Petrolatum. Zinc has limited high temperature use, is very reactive with moisture and even the grease thickeners used in anti-seize compositions. Its primary benefit is on aluminum fasteners where electrical current may be present. Since zinc is a good sacrificial anode it does not result in galvanic corrosion issues with the aluminum (or alloy). In wet applications and high temperatures, it can react with moisture and release hydrogen. Hydrogen tends to act as an acid as well as being flammable. The resulting zinc oxide has poorer anti-galling properties but can offer some other benefits.
Like most aspects of properly sealing bolted joint assemblies, the deeper a person dives into the subject the more complex it becomes. The selection of an appropriate anti-seize/thread lubricant is further complicated by confusing and incomplete product performance information.
By considering the fastener material, operational requirements and the information provided above on different anti-seize additives, an appropriate selection can be made as to which material should meet operational needs. However, additional testing should be done to establish a realistic K factor for use in calculating torque values. In addition, a simple test can be done to evaluate the level of protection that the anti-seize/thread lubricant will likely provide to the threaded components by using the anti-seize/thread lubricant when and placing the bolt material into a flange or other fixture and placing in an oven for a week at the maximum operating temperature the fastener will be exposed to. The torque needed to break apart the fastener will then provide a reasonable indication as to the level of protection that has been provided to the threads. Since no two anti-seize/thread lubricant materials will perform the same, evaluating different anti-seize/thread lubricant material options in this manner can significantly reduce the total cost of ownership while improving the reliability of bolted joint assemblies.
Valve engineers, valve repair shops, and end-users need to evaluate the impact of tighter sealing requirements not only on their valves designs but also their gland flange bolting. With the introduction of new valve production standards and plant-wide sealing expectations, a sealing approach that creates safe, accurate and reliable sealing performance should be adopted.