Queries commonly made up about the comparative resistance of Monel alloy 400 and Monel alloy K-500 against the stress corrosion cracking in the presence of HF, specifically in applications for pressure gauges utilized in refinery HF alkylation plants.

However Monel alloy 400 is found to be slightly more resistant to stress corrosion cracking in the aerated acid, the difference is not adequate to prevent a failure. These Monel alloy grades are exposed to strong attack in the vapor phase of HF in the presence of oxygen gas. This article emphasizes the need to prevent air in the vapor phase above the acid. As Hydrofluoric acid alkylation plants are carefully treated to prevent oxygen contamination, the slight different between these alloys should not be an influential aspect. It is preferred that gauges used for direct HF application are ordered to confirm the absence of residual water. If air cannot be removed from the process, a diaphragm seal should be utilized.

The elastic element is normally a thin walled material that of importance operates under the high stress conditions.

The complete scale pressure of gauge chosen should be almost double of the desired operating pressure, the highest pressure should not exceed 75% of the scale limit. The liquid filled and throttled or gauges with performance option should be followed on the pump discharge lines or where there is vibration noticed.

As the Bourdon tube of the common pressure gauges consists of a dead-end cavity, and as air increases the stress corrosion cracking, it is essential that a newly installed gauge should be empty and back-filled with an inert gas or liquid. It will prevent storage of air in the high stressed end of the tube. This step is often a standard part of the repair processes for HF alkylation units. It should not be ignored for just a nominal content of air is included in altering the gauge. An extent of air stored in a Bourdon tube will be in a position where it can cause lot of damage. If the lines cannot be cleaned, gauge needs to be changed, it could be evacuated and refilled with noble gas or liquid,capped or sealed as long as installation is done.

An evaluation of use of Monel alloys in hydrofluoric acid finds a slight difference between Monel alloy K500 and Monel alloy 400 in aerated HF vapors, there was significant overlap in the consequences. Cracks were initially observed in the four samples of Monel K-500 in two to six days whereas 25 samples of Monel alloy 400 introduced cracks in 4 to 15 days. Both alloys crack in the presence of aerated HF vapors.

Major difference among the alloys is generally because of the higher hardness of Monel alloy K-500, specifically in the age hardened condition.

Monel alloy 400 can be subjected to stress corrosion cracking in moist vapors in the presence of oxygen due to accretion of cupric fluorides. On the other side Monel alloy K-500 has higher strength, may be even more. While correct, duragauge and type 1259 gauges with Monel alloy K500 Bourden tubes are not age hardened. These are welded, over pressured and stress relieved. The age hardened tubes are utilized only for type 1082 test gauges and 30,000 psi process gauges.

Several pressure gauges with Monel K Bourdon tubes are given to refineries for over many decades, presumably for direct use on the hydrofluoric acid alkylation plants and other highly corrosive applications. However due to unfamiliarity about the maintenance of their gauges, it is probable that refineries only use the highly expensive Monel equipments in the corrosive applications. 

If air cannot be discarded in HF applications, a diaphragm seal should be utilized that remains for several years. Gold and platinum are the sole metals that are completely resistant to aerated and nonaerated HF and its vapors. onel Diaphragms gold plated to ASTM B488, electrodeposited coatings of Gold are used for engineering applications and checked to be porosity free to ASTM B735 porosity of gold coatings on metal substrates by Nitric acid vapor.     

Some studies show that Hastelloy alloys are more resistant, but these are not resistant to stress corrosion cracking and the test conditions were not much clear. Most of the corrosion information is about the uniform corrosion instead the more detrimental stress corrosion cracking. For those who choose the alternative materials or prefer another line of security in addition of Bourdon tubes diaphragm seals are introduced in the most corrosion resistive materials. Several combinations of diaphragms and lower housings are introduced in the welded Monel alloy 400 design.

For verification,a simple test is conducted to find whether a part is constructed of Monel alloy 400 or Monel K-500 alloy. At the temperature when Monel alloy 400 becomes magnetic is  40oF and 70oF whereas magnetizing temperature for Monel alloy K-500 is -150oF. If these alloys are kept in the domestic refrigerator and even at room temperature, Monel alloy 400 and 405 become partially magnetic. Monel alloy K-500 will not be magnetized.

The above test mainly is based on the Bourdon tubes, Monel gauges made of Monel alloy 400 sockets. Monel alloy 405 stated above is a free machining monel grade with added sulfur. The content of sulfur acts as sulfide stringers in the microstructure that improve the chip cracking. The refineries using the Monel alloy K-500 are satisfied with its performance for decades.