Stainless Steel

What is Stainless Steel?

Stainless steel is a metal alloy consisting of nickel, chromium, molybdenum, and other additives. Generally, the composition ratios of this alloy vary and depend on the target properties or use, however, a composition of at least 10.5% chromium and less than 1.2% carbon is required for it to be considered "stainless" [1][2]. As the name suggests, stainless steel is created to be more corrosion resistant to exposure compared to other materials such as carbon or alloy steels. As a result of its exposure resistance, high durability, aesthetics, and hygienic properties, stainless steel play a prominent role in a wide variety of industrial processes, and the production of consumer goods [2][3].

Applications of Stainless Steel

As a common alloy, stainless steel sees significant usage in engineering within different industries. It can be seen used in oil and gas, water utility, medical, electrical practices and much more. Through its extensive use, multiple variants and grade of stainless steel are available based on the different environments it's intended to be used in. For example, if stainless steel were to be used in a marine environment, a chloride variant would need to be used in order to resist corrosion from salty seawater. Furthermore, the application of stainless steel is able to alleviate costs due to corrosion damage; according to the US Federal Highway Administration, it is estimated that the US spends about 276 billion dollars a year on damage caused by corrosion. If more stainless steel were used instead of other pure metals, this amount would go down with increased reliability [9].

Applications of Stainless Steel Piping Within Chemical Engineering

One major application of stainless steel within Chemical Engineering is in piping. Piping allows for the transport of masses such as fluids and gases which are common in large chemical plants and processes. For example, pipes of various lengths and diameters transport raw materials, intermediary chemicals, and products between different condensers, mixers, evaporators, etc. As a result of a wide variety of pressures, temperatures, and exposure, the material that the pipe is made of is crucial [11].

Industrial Usages

Some of the most common applications of stainless steel pipes can be found in Chemical Engineering industries [11][12]:

• • • Food and Pharmaceutical Processing

    • Oil, Gas, and Energy

    • Environmental and Water Treatment Plants

    • Materials and Manufacturing

Chemical Plants and Refineries

Currently, stainless steel pipes are considered to be a mainstay of fluid systems with it's heavy usage in chemical and petrochemical plants [12]. It's chemical and mechanical properties makes stainless steel a better material than carbon steel; the thin layer of chromium oxide formed on the surface by the chromium of the alloy prevents corrosion from penetrating the surface. In addition, stainless steel is 100% recyclable making it much more environmentally friendly than alternatives - about all of stainless steel products, including piping, contains up to 60% of recycled material. The advantages of stainless steel for chemical plants and refineries are as described [12][13]:

Adaptability and Ease of Use:

• • • • Stainless steel equipment, tanks, and structural elements are easily to modify
      • Fluid systems using stainless steel welded pipes can be reworked while in operation
      • Rework with little risk of leakage (millibar per second or less)

Resistance to Corrosion:

• • • • Protected by most forms of corrosion, most common being oxidation
      • Best overall resistance to chemicals
      • Damaged oxide layer can be self-repaired; fresh exposed chromium is oxidized

Alleviates Operation Costs:

• • • • Higher initial purchase price
      • Lower long run costs from repairs

Food and Pharmaceutical

Stainless steel pipes sees heavy usage in food processing where cleanliness and sanitation is of the highest importance. Through fabrication, stainless steel pipes are smooth and crack free. This prevents the buildup of bacteria and microorganisms. The two most common types of stainless steel within the food industry are 304 and 316 stainless steel [12][14].

304-stainless steel

• • • • Excellent anti corrosion properties

      • Suitable for most food and biological products

      • Passivated surface compromised in the presence of chlorides

      • Chlorides can cause pitting

316-stainless steel:

• • • • Stronger resistance to chlorides compared to 304

      • Use for meat products

      • Use for products with mild salt content

Duplex-stainless steel:

• • • • Useful for stagnant or slow moving processes

      • Use for briny or salty foods

      • Applications where higher temperatures are required

Advantages and Disadvantages of Stainless Steel Pipes

Damage and Contamination Through Contact

The following can cause contamination if they come in contact with stainless steel [14]

• • • Affixing a non-stainless metals with stainless steel

    • Contact with carbon steel can cause contamination; spot corrosion

    • Hypochlorites, such as bleach

    • Sparks from welding and grinding; contaminate rust exposed surfaces

    • Long Term contact with other metals; galvanic corrosion

What To Use To Disinfect Stainless Steel

• • • • Nitric Acid

      • Iodine and Ammonium solutions

Ease of Keeping Clean

• • • • Smooth, non-porous, robust material

      • Majority of different cleaning processes

      • Can use Sodium Hydroxide clean-in-place system

Selection Based on Requirements

Connection Types/Joining Methods

The connection type for stainless steel piping is mostly dependant on the operation environment, and the materials that it comes in contact with. Welding, flanged, and threaded fittings are the most common for stainless steel applications. The various other types of connection methods can be found in Connection Types - Pipes page under Pipes and Tubing of the ChE subwiki [16][18].

Stainless Steel Pipe Sizing

Passivation

Passivation is a post fabrication treatment process for stainless steel to increase its corrosion resistance. The inner workings of passivation is not universally agreed upon, but functions with a thin layer of protective oxide on the passive stainless steel. This layer is less than 0.0000001 inches thick, and covers the entire surface of the stainless steel piece to protect it. Methods of which passivation can be achieved varies [17].

Cleaning

Cleaning of pieces is prioritized before passivation as corrosion can occur if parts of the surface is contaminated [17].

• • Cleaning of grease, oils, and coolant can be achieved through a commercial degreaser treatment
  • Dirt and other foreign debris should be wiped off
  • Thermal oxides can be removed through grinding or acid pickling

Nitric Acid

Nitric Acid With Sodium Dichromate

1. 5% weight Sodium Hydroxide at 160/180 degrees F (71/82 degrees C) for 30 minutes
2. Water rinse
3. 20% volume Nitric Acid + 3 oz/gal Sodium Dichromate at 120/140 degrees F (49/60 degrees C) for 30 minutes
4. Water rinse
5. 5% weight Sodium Hydroxide at 160/180 degrees F (71/82 degrees C) for 30 minutes (REPEAT STEP ONE)
6. Water rinse

References