Stepping Outside the Standards for Higher Corrosion Resistance

Stepping Outside the Standards for Higher Corrosion Resistance

Corrosion-resistant pumps are critical to many industrial processes.  While some duties may use plastics for pump construction, the mechanical limitations of this material mean that for most applications cast metals are used. Traditionally, high-alloy steels have been widely used with limited success in more severe duties. Around thirty years ago, when austenitic-ferric steels (duplex) became available, Sulzer Pumps was the first company in the world to use the material in seawater injection pumps. The following examines the latest high-alloy cast steel used by Sulzer Pumps for demanding industrial processes that delivers improved performance over duplex types.

Extended Pump Life
Pump manufacturers experience continuous pressure from users to extend the life of process pumps in order to benefit customers by reducing parts and labor costs and reducing the number of unplanned shutdowns. The key to life extension is to produce pump parts from erosion- and corrosion-resistant materials that nevertheless retain the properties essential for manufacturing the component. This balance is is often a compromise between providing good corrosion resistance and adequate strenth and hardness for the component.

Cast High-Alloy Steel
The production of steels used in pumps has traditionally been based on the American Society for Testing and Materials (ASTM) standard, which is widely accepted throughout industry. The more recent European standard, EN10283, also details steel grades intended for general applications. The European and ASTM standards contain many equivalent or nearly equivalent steel types. Typically, pump manufacturers use these standard grades for components because they source from commercial foundries. Sulzer Pumps is dedicated to continuous research on how to improve the performance of the materials it uses. In addition to these internationally recognized standard materialsm it also examines material outside of the industry standards. A good example of this is the completely austenitic, highly nitrogen-alloyed and extremely corrosion-resistant Avesta 654 SMO™ steel. This patented material is not included in either the American or European cast steel standards, so is not readily available from commercial foundries.

Benefits in the Foundry Process
In the past, the duplex-alloyed steels were usually easier to make than the completely austenitic corrosion-resistant grades. Production of the latter grades requires an argon oxygen decarburization (AOD) converter or, when melted in an induction furnace, raw material that has been pretreated in an AOD converter. Highly chromium- and molybdenum-alloyed steels must have both a low carbon content and increased nitrogen content; these conflicting needs are readily achieved using AOD technology. The low sulfur content achieved through this method lends additional corrosion-resistance to the material. Because of its modern foundry production facilities (Figs. 1 & 2), Sulzer Pumps Finland Oy has become one of only a limited number of licensed producers of 65SMO.

654 SMO Excels in Tests
Before committing to production of 654 SMO™, it was important to establish its superiority to most of the common alternative materials used in pumps. The material was therefore subjected to the following rigorous tests with positive results:

  • Critical pitting temperature: Results prove that 654SMO withstands pitting corrosion well. It shows a critical pitting temperature almost 40°C higher than for duplex steels.
  • Corrosion test: Samples were tested in an operational phosphoric acid reaction tank for a period of eleven days. In addition, a short-term abrasion test was also performed, using the reaction fluid to which the abrasive substance alumina had been added. Results indicate that 654SMO has similar abrasion resistance to the duplex steels and that it performs better in straight immersion tests.
  • Cavitation erosion test: The performance of 654SMO is clearly superior to the other materials (Fig. 3).

654 SMOExcels in Live Applications
Phosphoric acid is made by a "wet" process where natural phosphate rocks are immersed in sulfuric acid. Serious corrosion problems occur because of the aggressiveness of the industrial phosphoric  acid and abrasive nature of the raw pumped solution.
The Kemira plant located at Siilinjärvi (Finland) produces phosphoric acid. It chose Sulzer AHLSTAR WPP process pumps manufactured from 654 SMO™ for its production process. after three years operation a pump was inspected internally and proved to have withstood wear so well that the text "654 SMO" was still clearly visible on the impeller casting (Fig. 4).
A major manufacturer in Morocco put a Sulzer AHLSTAR WPP 61-600 pump (Fig. 5) manufactured in 654 SMO™ in its flash cooler circulation duty (after the reactor) on an experimental basis. After the completion of the six-month test period in this extreme corrosive application, the customer was so impressed by how well the pump had resisted corrosion damage that he immediately had three further units installed. 

State-of-the-Art Pumps
Sulzer process pumps manufactured from 654 SMO™ have class-leading resistance to erosion and corrosion attack. Sulzer Pumps can control the entire manufacturing process – from production of the initial melt and casting parts to selection of the most appropriate design features for a particular application. This ability enables it to provide exceptionally reliable pumping solutions with extended operational life that stand up to even the most arduous applications.

 

Riihmäki, Arto, "Stepping Outside the Standards for Higher Corrosion Resistance." Sulzer Technical Review. 1/2005: 13-15.

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