PTQ Q4 2023 Issue

Figure 3 Rotating cage test

Figure 4 Coupon test results

the temperature rises. A higher amount of the corrosion inhibitor is required at higher temperatures to maintain the protection layer. Careful consideration should be given to maintaining a temperature below the thermal decom- position point of the corrosion inhibitor. There are special filmers that have a good protective effect at high temper - atures of approximately 280°C (535°F) and high pressures of approximately 3.5 MPa. However, the usual dosing rates of 1-5 mg/L are not sufficient, and it may be necessary to dose more than 10 times to achieve the same protective effect at high temperatures. Rotating cage method Corrosion inhibitors can be tested very well with the rotat- ing cage method. In this test, untreated (blank) and treated C1018 steel coupons are fixed in a coupon holder (PTFE material) and immersed in a 1 wt% hydrochloric acid (HCl). The very corrosive HCl solution is heated to approximately 60°C (140°F) via a temperature-controlled water bath or a heating plate. The coupon holder is connected via an axis to a stirrer motor that rotates at high speed for 24 hours. Figure 3 shows an example of the structure of a coupon holder with four places for coupons. The coupons stand in a small tray of the coupon holder, each secured at the top with a screw (PTFE) to prevent them from falling out. A rotating cage test method can be used in a laboratory with minimal technical requirements. However, this testing method may not be as effective for water-soluble filmers. At the high rotational speed with high shear forces, the aqueous hydrochloric acid removes the water-soluble fil - mer more quickly from the metal surface, and the protective effect is thus impaired. This effect is not observed to the same extent with oil-soluble filmers. To treat the coupons, a diluted solution of a filmer is pre - pared beforehand, and the coupon is wholly immersed in it for 10 minutes. The treated coupon is then hung in a drying cabinet at 60°C for one hour and then fixed in the coupon holder. Each coupon has its serial number, and the weight is known. Therefore, one can later evaluate precisely how much metal has been removed by a corrosion attack. The rotating cage method is an excellent way to observe pitting corrosion.

Since untreated coupons are always tested together with treated coupons under exactly the same test condi- tions, it is effortless to compare the coupons with each other. At the end of the test, the coupons are removed from the coupon holder after 24 hours, washed and cleaned, and each coupon’s new mass is determined. In addition to evaluating the weight losses (mm/y or MPY), it is also possible to check whether pitting corrosion is pres- ent very well. Figure 4 shows the results of two test runs (2x four car- bon steel coupons) after 24 hours of treatment in hydro- chloric acid. In each test run, three different filmers (A, B, C) were tested and compared with an untreated sample. In the case of the two untreated coupons, it is easy to see that the metal surface has been attacked to a much greater extent. Pitting corrosion is visible at the edges in several places. The small red circle shows two of several partly deep holes on the coupon with the serial number 1857. Case study: Naphtha hydrotreater unit After a few weeks of operation, corrosion and fouling were regularly observed in three air-cooled condensers close in front of the accumulator drum of an NHT unit. The fouling material was observed to be corrosion products, ammonium chlorides, and ammonium bisulphides. To prevent further fouling, wash water was dosed before the air coolers, but

Infra-red camera:

Emissivity:

FLIR T335

0.69

Reected apparent temperature:

Object distance:

20.0˚C

5.0 m

Figure 5 Thermographic scans of NHT air cooled condensers

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PTQ Q4 2023