Erosion and Erosion-Corrosion in Process Equipment and Piping

Summary:

Erosion and erosion-corrosion are forms of damage that can occur in process equipment and piping exposed to moving fluids and/or catalysts. These damages can cause a localized loss in thickness in the form of pits, grooves, gullies, waves, rounded holes, and valleys. The metal loss rates depend on various factors, including the velocity and concentration of the impacting medium, the size and hardness of the impacting particles, the hardness and corrosion resistance of the material subject to erosion, and the angle of impact.

Preventing and mitigating erosion and erosion-corrosion involve improvements in design, changes in shape, geometry, and materials selection, and utilizing impingement plates and specialized corrosion coupons. Visual examination and specialized corrosion monitoring electrical resistance probes are among the inspection and monitoring techniques used to detect the extent of metal loss.

Electrical Resistance (ER) probes and instruments determine metal loss from corrosion or erosion by the electrical resistance method.

1. Description of Damage

a) Erosion is the accelerated mechanical removal of surface material as a result of relative movement between, or impact from solids, liquids, vapor or any combination thereof.

b) Erosion-corrosion is a description of the damage that occurs when corrosion contributes to erosion by removing protective films or scales, or by exposing the metal surface to further corrosion under the combined action of erosion and corrosion.

2. Materials Affected by Erosion-Corrosion

 All metals, alloys, and refractories are susceptible to erosion-corrosion.

3. Key Considerations

a) In most cases, erosion-corrosion occurs as a result of the combined action of mechanical erosion and chemical corrosion, with pure erosion (abrasive wear) being rare. Thus, it is important to consider the role of corrosion in contributing to damage.

b) Metal loss rates are influenced by several factors, including the velocity and concentration of the impacting medium (such as particles, liquids, droplets, slurries, and two-phase flow), the size and hardness of the impacting particles, the hardness and corrosion resistance of the material subject to erosion, and the angle of impact.

c) Softer alloys, such as copper and aluminum, may be more vulnerable to erosion-corrosion under high-velocity conditions due to their susceptibility to mechanical damage.

d) While increasing the hardness of the metal substrate is often considered a means to minimize damage, it may not necessarily improve resistance to erosion, especially if corrosion plays a significant role.

e) For each environment-material combination, there is usually a threshold velocity above which impacting objects may cause metal loss. Increasing velocities beyond this threshold results in higher metal loss rates, as shown in Table 1, which demonstrates the relative susceptibility of different metals and alloys to erosion-corrosion by seawater at varying velocities.

f) The size, shape, density, and hardness of the impacting medium also affect the rate of metal loss. 

g) Increasing the corrosivity of the environment can reduce the stability of protective surface films and increase the susceptibility to metal loss. Metal can be removed from the surface as dissolved ions or solid corrosion products that are mechanically swept from the metal surface.

h) Factors that increase the corrosivity of the environment, such as temperature and pH, can also increase the susceptibility to metal loss.

Table 1– Typical erosion-corrosion rates in seawater (API 571 Section 4.2.14)

4. Affected Equipment and Components

a) Erosion and erosion-corrosion can affect all types of equipment exposed to moving fluids and catalysts. This includes piping systems, such as bends, elbows, tees, and reducers, as well as downstream piping systems from letdown valves and block valves. Additionally, pumps, blowers, propellers, impellers, agitators, agitated vessels, heat exchanger tubing, measuring device orifices, turbine blades, nozzles, ducts, vapor lines, scrapers, cutters, and wear plates can be affected.

b) Erosion can be caused by gas-borne catalyst particles or particles carried by a liquid, such as a slurry. Refineries are particularly susceptible to this type of damage, as it can occur in catalyst handling equipment (valves, cyclones, piping, reactors) and slurry piping in FCC reactor/regenerator systems, coke handling equipment in both delayed and fluidized bed cokers (figure 1), and as wear on pumps (figure 2 and figure 3), compressors, and other rotating equipment.

Figure 1: Erosion of a 9Cr coker heater return bend (API 571 Section 4.2.14)

Figure 2: Cast iron impeller in untreated cooling water after four years of service (API 571 Section 4.2.14)

Figure 3: Close-up of Figure 2 showing both erosion-corrosion at the vane tips and pitting on the pressure side of the vanes (API 571 Section 4.2.14).

c) Hydroprocessing reactor effluent piping may be subject to erosion-corrosion by ammonium bisulfide, with the degree of metal loss dependent on several factors, including the concentration of ammonium bisulfide, velocity, and alloy corrosion resistance.

d) Crude and vacuum unit piping and vessels exposed to naphthenic acids in some crude oils may suffer severe erosion-corrosion metal loss depending on temperature, velocity, sulfur content, and TAN level.

5. Appearance or Morphology of Damage

a) Erosion and erosion-corrosion result in a localized loss of thickness, typically in the form of pits, grooves, gullies, waves, rounded holes, and valleys. These losses often exhibit a directional pattern.

b) Failures can occur quickly, making it crucial to address and monitor these types of damage.

6. Prevention and Mitigation Techniques for Erosion and Erosion-Corrosion

Erosion and erosion-corrosion can cause serious damage to equipment and structures, leading to costly repairs and downtime. Fortunately, there are several prevention and mitigation techniques that can be employed to minimize the impact of these damaging processes.

a) Design improvements are a crucial aspect of preventing erosion and erosion-corrosion. Changes in shape, geometry, and materials selection can all play a role. Examples of design improvements include increasing pipe diameter to decrease velocity, streamlining bends to reduce impingement, increasing wall thickness, and using replaceable impingement baffles.

b) Improving resistance to erosion is often achieved by increasing substrate hardness using harder alloys, hardfacing, or surface-hardening treatments. Erosion-resistant refractories, such as those used in cyclones and slide valves, have also been effective.

c) Erosion-corrosion can be mitigated by using more corrosion-resistant alloys and/or altering the process environment to reduce corrosivity. Techniques such as deaeration, condensate injection, or the addition of inhibitors can all help to reduce corrosion. It is important to note that increasing substrate hardness alone generally does not improve resistance to erosion-corrosion.

d) Heat exchangers can utilize impingement plates and tube ferrules to minimize erosion problems.

e) In applications where naphthenic acid corrosion is a concern, higher molybdenum-containing alloys can be used to improve resistance to this specific form of corrosion.

By employing these prevention and mitigation techniques, the impact of erosion and erosion-corrosion can be minimized, ensuring the longevity and reliability of equipment and structures.

7. Inspection and Monitoring

a) Metal loss can be detected through visual examination of suspected or troublesome areas as well as through ultrasonic (UT) or radiographic testing (RT).

b) In some applications, specialized corrosion coupons and on-line corrosion monitoring electrical resistance probes are used for monitoring purposes.

c) Infrared (IR) scans are employed to detect refractory loss in service.

8. Related Mechanisms

Specific terminology has been developed for various forms of erosion and erosion-corrosion in particular environments and/or services. These terms include cavitation, liquid impingement erosion, fretting, and other similar terms.

9. Conclusion

Erosion and erosion-corrosion can cause severe damage to equipment and facilities in the refining and petrochemical industry, leading to safety hazards, production losses, and increased maintenance costs. Effective prevention and mitigation measures are crucial to minimize the impact of these damaging mechanisms. This requires a combination of proper material selection, design improvements, and regular inspection and monitoring.

10. Future Scope

Further research and development in materials science and corrosion engineering can lead to more effective solutions for preventing erosion and erosion-corrosion in the refining and petrochemical industry. This includes the development of new alloys and coatings with improved erosion and corrosion resistance, as well as the advancement of non-destructive testing techniques for early detection of damage. Additionally, continued education and training for industry professionals on the importance of erosion and erosion-corrosion prevention and mitigation can help reduce the frequency and severity of incidents caused by these mechanisms.

11. References

1. ASM Metals Handbook, Volume 13, “Corrosion,” ASM International, Materials Park

2. ASM Metals Handbook, Volume 11, “Failure Analysis and Prevention,” ASM International, Metals Park.

12. Case Studies and Examples of Erosion and Erosion-Corrosion in Refining and Petrochemical Industry

Here are some examples of erosion and erosion-corrosion in the refining and petrochemical industry:

1. Erosion-corrosion in a crude oil unit atmospheric distillation column: In this case, the metal loss occurred due to the presence of naphthenic acid corrosion in the column's top tray. The damage was detected during an inspection, and the tray had to be replaced.
2. Erosion in FCC regenerator cyclones: The cyclones in an FCC regenerator were suffering from erosion due to the high-velocity flow of catalysts. The damage was addressed by replacing the existing cyclones with erosion-resistant ceramic cyclones.
3. Erosion-corrosion in hydroprocessing units: Ammonium bisulfide was causing erosion-corrosion in the effluent piping of hydroprocessing units. The problem was mitigated by using more corrosion-resistant alloys and altering the process environment.
4. Erosion in piping downstream of letdown valves: In a petrochemical plant, the piping downstream of letdown valves was experiencing erosion due to high-velocity flow of fluids. The problem was solved by installing replaceable impingement baffles.
5. Erosion-corrosion in FCC reactor feed nozzles: The nozzles in an FCC reactor were experiencing erosion-corrosion due to the high-velocity flow of catalysts. The damage was addressed by changing the material of the nozzles to a more erosion-resistant alloy.

By NTS

Corrosion Resisting steel - Grade 3CR12

3CR12 Technical Data

Summary

3CR12 is a chromium containing corrosion resisting steel developed as an alternative material of construction where the mechanical properties, corrosion resistance and fabrication requirements of other materials such as mild steel, galvanised or aluminised steel, aluminium or pre-painted steels are unsuited.

Originally 3CR12 was not included in any international specifications. However, a 12 per cent chromium steel developed from 3CR12 has been designated DIN type 1,4003 and ASTM/ASME 41003. The former has been incorporated into two Euronorm Standards viz. EN 10088 and EN 10028, 3CR12 conforms to the requirements of the above specifications and is multi certifiable to 3CR12, 1.4003 and 41003, due to its inclusion in the above specifications. 3CR12 vessels and tanks can be designed in accordance to BS5500, ASME, AD Merkblatter codes and the Euronorm design specification currently in preparation.

Although 3CR12 is recognised as the World's most specified 12% Chromium utility steel, it is by no means universal and should not be substituted for higher grades of stainless steel unless detailed corrosion testing has been carried out. Columbus Stainless can be consulted for advice in this regard.

3CR12 was designed as a corrosion resisting steel and, as such, will exhibit staining when exposed to aggressive atmospheric conditions. In applications where aesthetic appearance is important, it is recommended that 3CR12 is painted, or a higher grade should be used.

A long term atmospheric corrosion programme conducted over 20 years bv the CSIR has shown 3CR12 to have very good atmospheric corrosion resistance, Stainless steels, with their higher chromium contents, exhibited very low corrosion rates. Because of 3CR12's inherent corrosion resistance, it has been used successfully under wet sliding abrasion conditions such as found in the mining and bulk handling industries. In the case of mild or low alloy steels the presence of moisture in the solids being transported aggravates deterioration of the working surfaces. Not only does the surface rust wear away rapidly exposing bare metal to further corrosion, but corrosion of the working surface leads to 'hang-up' and interrupted flow. 3CR12 resists the corrosive attack and thereby improves flow and reliability, while extending the life of the solids handling equipment.

Although 3CR12 performs very well in corrosion-abrasion applications, no real benefit can be gained by using it under dry abrasion conditions. 3CR12 is not especially suitable under conditions of impact abrasion. (See: A Guide to the Use of 3CR12 in Corrosion Abrasion Applications).

3CR12 has been extensively used in aqueous environments, and has been successful in many applications involving exposure and/or immersion. It is important when using 3CR12 in aqueous environments that the decision be based on a thorough water quality analysis and microbial count. (See: A Guide to the use of 3CR12 in Water).

3CR12 is designed with ease of fabrication in mind and its composition and properties result in good forming, drawing, blanking and punching characteristics. The steel is easily welded by any of the recognised welding processes and should be post weld pickled/cleaned and passivated.

3CR12 has been included in SABS 0162 Part 4 - Code of Practice for the Structural Use of Steel. When replacing carbon steel with 3CR12, it is necessary to redesign mild and constructional steel components using the mechanical and corrosion resisting properties of 3CR12 in order to gain full advantage of potential material and fabrication savings.

This document covers black (hot rolled and annealed) 3CR12 as well as pickled (No1 and 2B) material, 3CR12 is available in the following finishes HRA, No 1, 2D and 2B, Whereas the latter three finishes can be used for all suitable 3CR12 applications, the HRA finish should only be used in applications where wet sliding abrasion occurs. It should never be used in immersion conditions The mechanical properties of the HRA material are similar to those of the No 1 finish material. A long term atmospheric programme conducted over 20 years by the CSIR has shown 3CR12 to have very good atmospheric corrosion resistance.

 

Properties of 3CR12
  Chemical Composition
 

%C	%Ni	%Mn	%Si	%P	%S	%Cr	Other
0.03   Max	1.5   Max	1.5   Max	1.0   Max	0.03   Max	0.03  Max	11.0 - 12,0	Ti  0.6 Max

  1. Mechanical Properties
 

Ultimate Tensile Strength (Transverse)	450 MPa Min
0.2% Offset Proof Strength   (Transverse)	< 6,0 mm thick  -  320 MPa Min  > 6,0 mm thick  -  280 MPa Min
Elongation (in 50mm)	< 6,0 mm thick  =  20% Min
	> 6,0 mm thick  =  18% Min
Hardness	< 12.0 mm thick  -  220 Brinell Max
	> 12.0 mm thick  -  250 Brinell Max
Charpy Impact (Ambient temperature)	35 J/cm2 Min

2. Fatigue

Extensive testing has shown that 3CR12 behaves in a similar manner to constructional steels such as BS4360 Grade 43A in terms of fatigue. Accepted procedures when desgning for fatigue loaded structures should be followed. BSBS7068 can be used.
 
3. Physical Properties

At Room Temperature.
Density		7 740 kg/m3
Elastic Modulus (Tension)		200 GPa
Specific Heat Capacity		478 J/kg K
Thermal Conductivity	@ 100oC	30.5 W/m K
	@  500oC	40.0 W/m K
Electrical Resistivity		66 x 10-9Wm
Co-efficient of	0-100oC	11,1 mm/mK
thermal expansion	0-300oC	11.7 mm/mK
	0-700oC	12.3 mm/mK
Melting Range		1'430 - 1'510oC
Relative Permeability		Ferromagnetic

4. Corrosion Resistance

3CR12, with chromium as its major alloying element, is not intended as a material for use in contact with process solutions such as acids, salts, etc.  It is more suited to applications involving ancilliary equipment on process plants such as cable racking, stairways, flooring, handrailing, etc.  3CR12 is a "corrosion resistant" rather than "stainless" steel and as such, will tend to form a light, surface rust or discolouration when exposed to aggressive environments.  This patina is superficial and does not affect the mechanical properties of the steel.
Should aesthetic or hygienic qualities be of prime importance, stainless steels rather than 3CR12 should be considered, although 3CR12 can be successfully painted with a number of paint systems.

Aqueous Corrosion

It is recommended that consultations be held with Columbus Stainless technical staff on the use of 3CR12 in water.
At the design stage, efforts must be made to avoid crevices, sedimentation, stagnancy, high operating temperatures etc., as these facts will have a negative impact on the performance of the steel.
3CR12 is not recommended for use in hot water systems unless detailed testing has previously been carried out.

Atmospheric Corrosion

A long term atmospheric corrosion programme conducted over 10 years by the CSIR has shown 3CR12 to have very good atmospheric corrosion resistance.  Data on the performance of various materials at different test sites is available from VRN Technical staff.

5.  Fabrication of 3CR12

Note:  A detailed 3CR12 fabrication guideline is available from Columbus Stainless.

Cutting

For general fabrication requirements, the most effective cutting methods are:                       
                                                         

Abrasive disc	- use dedicated discs
	- avoid overheating
	- vitrified or resinoid aluminium oxide discs  recommended
Plasma	- oxygen-free nitrogen is the most economical primary cutting gas.    (Other gasses can be used)
	- heat discolouration must be removed prior to use in a corrosive   environment
Guillotine	- use well sharpened and correctly alligned and set blades to avoid sheared breaks and rollover.
	- capacity of guillotine (rated in terms of mild steel thickness) must be   downrated by 40% of 3CR12.

Forming

It is important to note that due to the higher proof strength of 3CR12, more power is required for most forming operations, than would be needed for mild steel.
When bending 3CR12 it is important to maintain a minimum inner bend radius equal to twice the material thickness.  Reverse bending at ambient temperatures is not recommended - the bend area should be preheated to +- 150^oC .  Edge cracks can be avoided by placing the cut face on the outside radius of the bend and the sheared face on the inside.  This type of cracking can also be prevented by grinding the outside radius point of bending into a rounded profile, thus eliminating the natural stress concentration point.

Welding

Manual metal arc, metal inert gas and tungsten inert gas are the common procedures used.  All welding procedures must ensure that heat inputs are kept to a minimum.  Down-hand welding is the preferred welding position and bead runs rather than weaving should be used.  Austenitic stainless steel filler metals such as AWS ER 309L, 308L, or 316L should be used.
In order to ensure adequate corrosion resistance in weld zones, it is necessary to remove all heat tint by pickling or by some mechanical means and passivating with a cold 10% nitric acid solution after cleaning.  Thorough washing with clean, cold water pickling and passivating is essential.

Machining

In the annealed condition, 3CR12 has machining characteristics similar to AISI 430 i.e. a machinability rating of 60.  The reduced extent of work-hardening compared to austenitic stainless steel eliminates the need for special cutting tools and lubricants.  Slow speeds and heavy feeds with sufficient emulsion lubricant will prevent machining problems.

Fastening

Where 3CR12 sections are to be bolted, stainless feel fasteners such as type 304 or 431 are preferred.  If bolted structures are to be used in humid or wet environments, it is strongly recommended that compressible, non-absorbant gaskets such as rubber be used.

Thermal Processing

Annealing
3CR12 is supplied in the annealed condition, its softest and most ductile state.  After severe cold forming operations or after hot forming operations above 750^oC, annealing may be required.  Annealing is carried out at 700-750^oC  followed by air cooling.
Soaking times are 12 hours per 25mm section.

Stress Relieving
Stress relieving is not recommended for 3CR12.  If it is essential, temperatures of not more than 450^oC  should be employed.

Hot Forming
Any hot forming should preferably be conducted at temperatures below 750^oC. The recommended temperature range is between 600^oC and 700^oC and annealing should be performed after forming.

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