Under-deposit corrosion is a type of corrosion that occurs beneath localized deposits of impurities or contaminants on the surface of a metal. These deposits create a microenvironment where the conditions for corrosion, such as oxygen depletion, pH changes, and concentration of aggressive ions, are different from the surrounding area. As a result, corrosion can initiate and progress rapidly underneath the deposit, leading to localized pitting, cracking, or other forms of degradation.
What is Under Deposit Corrosion?
Under deposit corrosion is a generalized term used for the active corrosion of bottom sections of process equipment and low velocity section of the process piping, under the substantial amount of accumulated semisolids. Usually the deposited corrosive sludge and the salts generated in the process are not removed from the process equipment due to inadequate design condition.
These deposits with the addition of moisture create various kind of corrosive environment. The localized corrosion cells are formed under these deposits which act as the anode with respect to the surrounding metal and some time the deposits themselves and cause the aggressive localized corrosion. The example is the formation of HCl under the deposited chloride salts of ammonia and other non-metals inside the process streams.
Under deposit corrosion is difficult to control unless the deposits are mechanically removed. The depositing some times are removed by jetting (wash water or steam) in the process equipment like tanks and separators, or by scarping in the scrapable flow lines.
The primary under deposit corrosion mechanisms for refining equipment are as follows.
a) Ammonium Chloride Corrosion.
Ammonium chloride salt is formed in various refining processes. These salts precipitate at ambient temperature and deposit at the low laying locations with stagnant flow conditions. Sever under deposit type corrosion takes place which could be either localized in small areas of spread out in larger areas under bigger deposits.
The primary contributors to the corrosion are also the HCl and other acidic components present in the deposits. UT scanning and Profile RT are the best tools to detect this corrosion. Further details on this damage mechanism can be seen from API‐RP‐571 Para 5.1.1.3.
b) Ammonium Bisulfide Corrosion
In most of the processes the ammonia is generated from the nitrides in the feed stock. Sulfur can be originated from the desulfurization processes. The sulfur and ammonia combine chemically and generate ammonium bisulfide (NH4HS) which is highly corrosive alkaline salt.
Ammonium bisulfide induces aggressive generalized as well as localized corrosion at the concentration above 2% by weight. Most of the corrosion is found at the impingement areas of the piping holding high velocity and turbulent flow. However in case of low velocities the precipitation occurs readily. The deposited salts of ammonium bisulfide induce
aggressive under deposited corrosion.
Carbon steel is least resistant and can experience high corrosion rates. 300 series stainless steels, duplex stainless steels, aluminum alloys and nickel base alloys are relatively resistant, depending on ammonium bisulfide concentration and velocity. UT scanning and profile radiography on the impact points (such as direction changes, valves and
reducers etc), are the best on‐stream inspection tools for ammonium bisulfide
corrosion.
Common locations for corrosion are in the inlet header box of the reactor effluent exchangers, piping elbows and welds with excessive I.D. weld metal penetration. Corrosion rates as high as 200 mpy have been observed in concentrated ammonium bisulfide solutions. Proper design of the piping to control the turbulence in the flow, avoiding high velocities and extensive water washing to keep the concentration of the salt down, are major methods to mitigate the Ammonium bisulfide corrosion.
Ammonium Bisulfide Corrosion can also be easily categorized in the erosion corrosion group as erosion is the dominating factor in this damage mechanism. Erosion corrosion is discussed in details in Para 1.3 of this section. Further details on this damage mechanism can be seen from API‐RP‐571 Para 5.1.1.2.
c) Chloride Salt deposits.
There is a range neutralizer used in the process industry to neutralize the acids formed as byproducts of main process. The chloride salts are formed as the result of neutralization action. In some cases the chloride are added into the streams for chlorination purposes or in order to activate the catalyst in the catalytic reformation process.
These chloride salts when cooled down in the downstream ends, often deposit in the equipment and low laying piping with inadequate flow. Along with the inherent moisture in the process streams, these salts cause aggressive under deposit corrosion in carbon steel and stress cracking in the stainless steel. The chloride stress corrosion cracking is further
discussed in Para. 3.1.3 of this section.
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What are Mechanism of Under-Deposit Corrosion?
1. Formation of Deposits: Deposits of various substances, such as scale, rust, organic matter, or other contaminants, accumulate on the surface of the metal. These deposits can form due to factors like water impurities, microbial activity, or environmental exposure.
2. Creation of Microenvironments: Underneath the deposits, localized changes in the environment occur. Oxygen depletion, pH changes, and concentration gradients of aggressive ions may develop within the crevices or beneath the deposits.3. Initiation of Corrosion: The altered environment beneath the deposit creates conditions favorable for electrochemical corrosion reactions to occur. In the absence of sufficient oxygen or in the presence of corrosive species, the metal begins to corrode.
4. Localized Attack: Corrosion progresses rapidly underneath the deposit, leading to localized pitting, crevice corrosion, or stress corrosion cracking. The corrosion damage may penetrate deeply into the metal substrate, weakening its structural integrity. -
What Factors are Influencing Under-Deposit Corrosion?
Composition of Deposits: The nature and composition of the deposits play a significant role in determining the severity of under-deposit corrosion. Deposits containing corrosive substances or promoting aggressive conditions exacerbate the corrosion process.
Environmental Conditions: Factors such as temperature, humidity, oxygen availability, and the presence of corrosive species in the surrounding environment influence the rate and extent of corrosion underneath the deposits.
Metal Properties: The susceptibility of the metal to under-deposit corrosion depends on its composition, microstructure, and surface condition. Certain metals and alloys are more prone to localized corrosion than others.
Hydrodynamics: Fluid flow patterns and turbulence in the environment affect the formation and stability of deposits on the metal surface. Stagnant areas or regions with low flow velocity are more prone to deposit accumulation and under-deposit corrosion. -
What are Prevention and Mitigation?
Regular Inspection and Cleaning: Periodic inspection and cleaning of metal surfaces to remove deposits and prevent their accumulation can mitigate the risk of under-deposit corrosion.
Surface Treatment: Applying protective coatings, inhibitors, or passivation treatments to metal surfaces can provide a barrier against deposit formation and corrosion initiation.
Optimization of Operating Conditions: Controlling environmental factors such as temperature, pH, and fluid composition can help minimize the formation of aggressive deposits and mitigate under-deposit corrosion.
Material Selection: Choosing corrosion-resistant materials or alloys for applications exposed to aggressive environments can reduce susceptibility to under-deposit corrosion.