Types of deposits and corrosion in MED desalination water

In MED desalination with seawater, subsoil corrosion is like a ticking time bomb. The scale appears to be calm, but underneath, the metal is being destroyed.

In this expert guide, we examine in full detail all the common types of scale in MED desalination (calcium carbonate, calcium sulfate, magnesium hydroxide, silica) and the most dangerous type of corrosion, subsoil corrosion.

This article is based on the experience of implementing dozens of real projects in Iranian coastal refineries and power plants, where high-chloride seawater creates a deadly combination of scale and corrosion.

Why read this article?

✅ Identify the 4 main types of MED deposits and their physical and chemical characteristics

✅ Understand the mechanism of subsedimentary corrosion and why it is so common in MED with seawater

✅ Learn about accurate detection methods (XRD, SEM, ICP) for each type of deposit and corrosion

✅ Receive specialized treatment solutions with proprietary formulations

✅ Access to real projects implemented in Iranian industries

Why is seawater a deadly combination for the MED?

Seawater in the Persian Gulf has certain characteristics that exacerbate sedimentation and corrosion:

Deadly combination:

Sediment + chloride + high temperature → creates a local corrosive environment under the sediment → subsediment corrosion → sudden leak!

Types of deposits in MED desalination — detection and risks

1. Calcium carbonate (CaCO₃) —the most common deposit

Origin :

(at high temperature and pH > 8.5)

Ca²⁺+ HCO₃⁻ → CaCO₃↓

Appearance : white or gray — brittle — similar to chalk

Hazards :

• Reduces heat transfer → increases steam consumption
• Creates a substrate for subsediment corrosion

Chemical detection :

• Dissolution in acid with bubbling (CO₂)
• XRD analysis : calcite peaks

2. Calcium sulfate (CaSO4) — Hardest precipitate

Origin :

(at high concentration and temperature > 70°C)

Ca²⁺+ SO₄²⁻ → CaSO₄↓

Appearance : White or yellowish turbid — very hard and sticky

Hazards :

• Strong adhesion to metal — difficult to remove mechanically
• Resistant to common acids

Chemical identification:

• Insoluble in HCl — slightly soluble in hot HNO3
• XRD analysis : Gypsum peaks

3. Magnesium hydroxide (Mg(OH)₂)—caused by high pH

Origin :

(at pH > 10.5)

Mg²⁺+ 2OH⁻ → Mg(OH)₂↓

Appearance : White, gelatinous, sticky

Hazards :

• Formation of insulating layer — reduced heat transfer
• Adsorption of chloride — increased subsurface corrosion

Chemical identification :

• Dissolution in acid — no bubbles
• SEM-EDS analysis : presence of Mg and O

4. Silica (SiO₂)—most dangerous precipitate

Origin : dissolved silica → SiO₂↓(at high temperature and alkaline pH)

Appearance : glassy or opaque — very hard — similar to glass glue

Hazards :

• Strong adhesion — non-removable with common acids
• Severe reduction in heat transfer — even in thin layers

Chemical identification :

• Insoluble in mineral acids
• SEM-EDS analysis : Si identification — XRD analysis : quartz peaks

Under-Deposit Corrosion — The Main Threat to Seawater MED

Why is under-deposit corrosion common in seawater MED?

• Deposits form on the metal surface
• Chloride accumulates under the deposit
• High temperatures (60–70°C) increase the corrosion rate
• Creates a local acidic environment (pH < 2) under the deposit → Rapid metal degradation

Appearance:

• Localized corrosion under the deposit layer
• Deep and sharp pits — usually in evaporator tubes
• No warning — until leakage

Diagnosis:

• Inspection after deposit removal — Localized corrosion under the deposit
• SEM-EDS analysis: Presence of Cl and O at the corrosion site
• ICP analysis:High chloride concentration in concentrated water

Real project:

At the 8th South Pars Refinery, deposit and subdeposit corrosion in MED pipes were identified by SEM analysis and treated with a proprietary Mitreh formulation. Preventing leakage with very high losses. (Signs of need for MED chemical cleaning)

Accurate Diagnosis Methods — Advanced Laboratory Analysis

Advanced laboratory methods are essential for accurate diagnosis of the type of deposit or corrosion:

✅ Laboratory Summary:

• XRD → To identify the type of deposit
• SEM-EDS → To identify subsedimentary corrosion
• ICP → To identify chloride and silica concentrations in water

Quick Diagnosis Checklist — What is the type of deposit or corrosion?

Practical solution: Abrizan Industrial Research Company — Accurate diagnosis and specialized treatment

If you are facing any of these deposits or corrosion, Abrizan Industrial Research Company, with its well-equipped laboratory and expert technical team, is ready to be by your side.

1. Specialized laboratory — Accurate diagnosis with XRD, SEM, ICP

• Sending a sample of the deposit or corroded metal
• Complete analysis in less than 72 hours
• Specialized report with a proposal for a specific formulation

2. Specialized products of the Mitreh brand — Treatment of any type of deposit and corrosion

• Special anti-scaling for thermal desalination packages
• Special anti-scaling for brackish water
• Special anti-scaling for seawater
• Oxygen remover
• Special sedimentation vectors for removing corrosion products
• Special sedimentation vectors for removing hydrocarbon compounds
• Special anti-corrosion for MED desalination

3. Field Chemical Washing Services

• Execution of washing with proprietary formulation — under laboratory supervision
• Pre and post washing report — with ICP analysis of washing water

Real projects:

• Project 1 : MED washing at South Pars 12th Refinery — 85% production capacity recovery with MDA0102 mitreh
• Project 2 : Descale 8 MED units at South Pars Gas Complex — Removal of deposits with cold mitreh
• Project 3 : Descale thermal desalination package at South Pars 3rd Refinery

✅ Final Conclusion:

In MED desalination with seawater, the combination of scale and chloride is a serious threat. Accurately identifying the type of scale and corrosion is the first and most important step for proper treatment. Using advanced laboratory methods (XRD, SEM, ICP) will help you choose the best chemical solution without guesswork.

Abrizan Industrial Research Company with the Mitreh brand, relying on a specialized laboratory, formulated products and a professional executive team, is ready to increase the efficiency of your system, reduce operating costs and increase the life of the equipment by up to 2 times.

Contact our sales department for free expert advice.