Under Deposit Corrosion: The Hidden Enemy in Your Cooling Tower

You have invested in a high-quality system and implemented water treatment, yet you still encounter mysterious damage and leaks? It could be that Under Deposit Corrosion (UDC) is the culprit. This type of corrosion is extremely dangerous because it operates silently, hidden beneath a layer of deposits, and is often detected only when the damage is already severe.

What is Under Deposit Corrosion (UDC)?

Under Deposit Corrosion is a highly aggressive localised form of corrosion that occurs beneath layers of deposits (such as sludge, scale, corrosion products, biofilm, or other debris) adhering to metal surfaces. This corrosion is particularly dangerous because it is localised and concentrated, which can lead to perforation (holes) and sudden equipment failure even though the rest of the metal surface appears to be fine.

How Does It Happen? (The Hidden Mechanism)

The UDC process is an electrochemical process that occurs under hidden conditions. Here is a simple explanation:

1. Deposit Formation: First, a deposit layer forms on the metal surface. This deposit can originate from:

   • The corrosion product itself (rust)

   • Mud, soil, or dust carried by water

   • Scaling (calcium carbonate, silica)

   • Biofilm or microbiological growth (MIC)

   • Organic dirt

2. Creation of an Oxygen Concentration Cell: This is the crux of the problem. The deposits block the diffusion of dissolved oxygen to the metal surface underneath. As a result:

   • The area underneath the deposits becomes oxygen-deficient (anodic site).

   • The area surrounding the deposits that is exposed to water has an abundance of oxygen (cathodic site).

3. Corrosion Acceleration: In this electrochemical cell, the metal in the anodic area (under the deposit) will oxidise and dissolve as ions, causing very rapid and deep corrosion. Meanwhile, the cathodic area (around the deposit) is protected. This potential difference creates a very aggressive and focused corrosion attack at a single point.

Main Causal Factors

1. Poor Water Quality:

   • Water with high total suspended solids (TSS) that form sludge.

   • Water with high hardness that triggers scaling.

   • Water conditions that allow microbial growth (algae, bacteria).

2. Lack of Filtration: Absence or inadequacy of a side-stream filtration system to filter fine particles from circulating water.

3. Inappropriate Chemical Treatment Programme:

   • Insufficient dosage of dispersant to prevent sedimentation.

   • Corrosion inhibitor programme that is ineffective in protecting under deposits.

   • Biocide treatment that fails to control biofilm.

4. System Design: Areas with slow or stagnant flow (dead legs) that facilitate sedimentation.

How to Detect It? (The Silent Killer Must Be Found)

Detecting UDC is challenging because it is hidden, but it is not impossible:

1. Visual Inspection (during shutdown): Open and inspect the inside of pipes, heat exchangers, and basins. Look for tubercles (deposit mounds) and when cleaned, deep holes or pits will be found underneath.

2. Non-Destructive Testing (NDT):

   • Ultrasonic Testing (UT): The most common and effective technique. UT can measure the wall thickness of pipes/tanks. A locally and significantly reduced thickness reading in an area may indicate underlying corrosion attack.

   • Radiography Testing (RT): Can see damage beneath the surface, excellent for finding pitting beneath deposits.

3. Corrosion Monitoring: Installation of corrosion coupons or online corrosion meters can provide general corrosion rate data. High corrosion rates can be an indication of a problem, although they do not specifically indicate UDC.

4. Removing Deposits: Physically cleaning deposits and inspecting the metal surface underneath is the most conclusive method.

Prevention and Treatment Solutions by PT. Jaya Sakti Internusa

Preventing UDC is far better and cheaper than repairing it.

1. Water Pretreatment Optimisation:

   • Install a side-stream filter or automatic backwash filter to continuously filter suspended particles from the system water.

   • Use a water softener or reverse osmosis if the makeup water has high hardness.

2. Comprehensive Chemical Treatment Programme:

   • Dispersant Polymers: To keep fine particles suspended in water and prevent them from settling.

   • Specialised Corrosion Inhibitors: Inhibitor formulations capable of forming a strong protective layer even on contaminated surfaces (under deposits).

   • Effective Biocides: To control the growth of biofilm that acts as a deposit adhesive.

3. Proper Design & Operation:

   • Avoid stagnant flow areas in system design.

   • Perform regular cleaning and flushing, especially after shutdown.

4. Regular Monitoring and Inspection:

   • Schedule regular visual inspections and UT scanning during shutdown periods to detect problems early.

   • Monitor water parameters (TSS, iron, turbidity) regularly.

Don't Let Hidden Corrosion Ruin Your Investment

Under deposit corrosion is a real threat that requires a proactive approach. Our team of water treatment experts can help you design a comprehensive defence strategy, from water analysis and filtration system recommendations to targeted chemical treatment programmes.