Conformal Coatings for Oil & Gas Sensors
Posted by Sean Horn
Friday, May 8, 2020 8:00
@ 8:00 AM
Oil & Gas industry makes use of sensors which face extremely high temperature and high pressure (HTHP) environments as in downhole drilling (>200°C and 30 kspi). In US natural gas supply lie in reservoirs below 15,000 ft. Wells at these depths pose environmental challenges of drilling due to the temperature, pressure and gasses. Increasing the lifetime of drilling equipment and possibility for recalibration of sensors that drift at such depths would save millions of dollars if they can be preserved under these HTHP environments. The packaging of sensors is a key element in providing this kind of protection.
Table: Sensors used in Oil & Gas Industry
|Vision based Non-contact Sensing|
Studies conducted on electronic sensors showed that the use of a conformal coating is beneficial for improving their lifetime if the device will be exposed to a corrosive environment such as sulfur or chlorine and thermal cycles, . It was shown that the Poly Urethane (UR) can provide an excellent protection against sulfidation when used in a high-sulfur environment during its service life at 60 °C’s . 27 μm UR and 17 μm Parylene C was studied for the corrosion of metallizations under various environments including H2S, Cl2, NO2, SO2 at 50 °C’s. It was reported that a uniform thickness coverage along the edges of the terminals may result in better performance with both coating types. And, it is advised that focusing on terminal coverage as well as the flat surface thickness of the conformal coatings will result in a uniform protection . Polyurethane conformal coatings can withstand thermal shock cycles of -65 °C to 125 °C. On the other hand Parylene XY variants (Parylene C, Parylene N, Parylene D, Parylene AF-4, or other variants) withstand higher temperatures with better conformal coating properties and better corrosion protection which makes them very competitive as conformal coatings for HTHP environments. Both UR and Parylene are versatile materials and are suitable for different applications. For example, Thick UR coatings on electronics with uniform coverage can be used in the protection of electronic parts which require thermal and corrosive resistance in the vicinity of 125 °C’s. UR provides protection against impact due to its high hardness. On the other hand, Parylene XY makes a pinhole-free protective coating at lower thicknesses (a few microns). Among Parylene variants Parylene AF-4 (350 °C) has the highest thermal durability which is followed by Parylene-C (100 °C) and Parylene-N (80 °C). Also, Parylene AF-4 has the lowest dielectric constant (2.17 @ 1 MHz) among the three which makes it more attractive. It can be used as a dielectric/insulating layer where high temperature exposure is required. Because, Parylene variants are deposited via chemical vapor deposition they have a very dense packaging which prevents corrosive gas penetration for a long time compared to other types of conformal coatings. The removal of Parylene is very hard and for drill hole sensing protective purposes they are suitable as durable coatings. Polyurethanes have high chemical resistance and they can be removed using stripping agents, on electronic parts urethane conformal coatings are offered as an easy to apply solution. UR can be applied by spaying, dipping or brushing. Both coatings can be used to prevent damage due to mechanical shock. However, because UR is a thick coating it can also break easier compared to its thinner and more flexible counterpart Parylene. Especially, in Oil and Gas industry drill hole wells both vibration and shock are inevitable this also points us to the use of Parylene. And, Parylene resist room temperature chemical attack and is insoluble in all organic solvents up to 150° C .
Table of Comparison:
Table: Parylene Types and Properties
|Property||Unit||Parylene XY||Parylene C||Parylene F –AF4||Polyurethane|
|Durable Heat Resistance||°C||80||100||350||up to 125|
|Rockwell Hardness||GPA||R85||R80||R122||80-95A (Shore)|
|SWELLING: benzene, chloroform, trichloroethylene, toluene.
2-propanol, ethylene glycol, water.
|No oxidation||Soluble in solvents (DMF, DMSO, etc.)|
|Water Absorption (MWTR)||0.01%/24 hour||0.06%/24 hour||0.01%/24 hour||Higher 0.07-0.20%|
|Advantages||ü Constant dielectric coefficient at all frequencies
ü High dielectric strength
ü Less wear (low friction coefficient.)
|ü Low gas permeability
ü High Chemical Resistance
|ü Sub-micron coverage 
ü High thermal resistance
ü Easy to apply
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