Connections Surface treatments

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In order to ensure the functional efficiency of a connection, even after multiple make-up operations, surface treatments are applied to the box, the pin or to both. The surface treatments are applied to improve:

  • the resistance to galling;
  • the sealing capability;
  • the resistance to corrosion.

Below the effect of the different surface treatments on these phenomena is highlighted. First a brief description of the most common treatment is given.

1 Process descriptions

The following surface treatments are used:

Phosphating Phosphating is the process by which the steel is dipped into a boiling, almost saturated, acidic solution of metal phosphate, for instance zinc phosphate or manganese phosphate. Reaction of the steel with the solution causes metal phosphate to precipitate and adhere on to the steel surface.

Lacquer coating Anti-friction lacquer, for instance a molybdenum disulphide, is sprayed evenly on pin and box surfaces, which have been heated to harden the lacquer in a short time. In order to provide a good base for the adhesion of the lacquer, the sliding surfaces are roughened by means of grit blasting.

Oxalating Oxalating is the process in which a thin layer of oxalate is applied to steels with a high chrome or nickel content, by dipping the steel into a hot oxalic acid solution. The process is more or less similar to phosphating.

Electrochemical treatments Electrolytic plating can be done with a wide variety of materials such as: gold, silver and its alloys, cobalt, nickel, chromium, copper, zinc, cadmium and tin. The materials deposited by the plating tend to be more finely grained, are usually harder and more brittle than wrought materials.

Ion deposition For high-alloy materials and nickel based alloys the application of a metallic surface coating via ion implantation process is used. Normally, in such case the pin is left untreated. During the ion implantation process, a microscopically thin film of ions from dissimilar metals, such as gold, chromium, copper or aluminium, is diffused into the base material.

Grit blasting, glass-bead peening Grit blasting and glass-bead peening are performed on the threaded area as well as on the sealing area. The treatment converts the smoothly machined surfaces into surfaces containing pits for oil and grease retention.

2 Effect on galling resistance

Bare tapered threads are extremely prone to galling during power make-up. This galling starts with the occurrence of high contact pressures, which are able to destroy the lubrication film and thus causing a direct metal-to-metal contact. The following treatments are given to improve the galling resistance of the connection during make-up.

Phosphating The phosphate layer improves the compound retention and surface hardness. Phosphating is commonly applied to carbon steel connections which are less susceptible to galling problems than more highly alloyed steels. For these materials other techniques have been developed. There is some evidence that manganese phosphate offers better galling resistance than zinc phosphate.

Lacquer coating Lacquer coatings have been applied on connections made from high-alloy steels. However the number of problem-free make-up operations which could be achieved was limited.

Oxalating This process is applied to high alloy steels. The oxalate layer improves the surface hardness and the capacity to hold on to a thin layer of compound.

Electrochemical treatments The most common electrochemical plating treatments used for casing connections are: Copper plating: The most attractive electrochemical surface treatment in terms of galling resistance is copper plating to a thickness of approximately 10 mm.- Tin plating: Tin plating is an excellent coating material. However, too high contact stresses can pulverise the tin layer and destroy the inter-metallic bonding, effecting the reusability of the connection. Furthermore, there is the risk of liquid metal embrittlement that might occur in the base material at temperatures above 350°F (175°C). Zinc plating: Conventional zinc plating baths produce fine-grained, smooth and brittle deposits, which however have poorer lubricant-retention properties than tin plating.

Ion deposition It is considered that the process has some potential for reducing the risk of galling for critical applications, particularly for high alloy tubular.

Grit blasting, glass-bead peening As galling may be promoted by too fine a surface finish, it is thought that a slightly rough surface aids lubrication by the thread compound trapped in the surface indentations.

3 Effect on sealing capability

The general consensus is that the best surface treatment for Premium connections in terms of galling is copper coating. However, it has been reported that tin plating is more effective than copper in improving the sealing capability of API connections. This is apparently the result of the greater thickness of tin plating which can be achieved, thereby providing an additional filler for the small clearances between the pin and box threads.

4 Effect on corrosion resistance

Find below the effect of the different surface treatments on the corrosion resistance.

Phosphating The effects of phosphating treatment in inhibiting corrosion are attributed to the ability of the phosphate layer to hold compound. If the connections are cleaned with a solvent and the compound layer is not replaced, the protection is largely lost. It is common practice for some manufacturers to apply phosphating to the field pin ends only as an anti-corrosion treatment. However, it is recommended that the same surface treatments should be applied to both ends of the tubular.

Oxalating The effect of oxalating is similar to the effect of phosphating.

Copper plating A layer of copper on top of the steel will increase the risk of bimetallic corrosion. The potential for bimetallic corrosion occurring downhole is, however, limited since oxygen would be required. During storage and transit, it is important to provide adequate protection against the ingress of moisture.

Tin plating The electrochemical potential between steel and tin is small and the risk of bimetallic corrosion can practically be ignored, therefore tin plating gives good protection against general corrosion. However, the designer should be aware of possible liquid-metal embrittlement corrosion.

Zinc plating Zinc is anodic to steel and sacrifices itself to protect the steel.

Comments  

#1 ENRIQUE BALLESTEROS 2015-10-06 16:42
what´s the standard for the specs and process of Tin Plating?
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