Reasons for Rusting at the Tube Sheet Weld Joint of a Shell and Tube Heat Exchanger
Rusting (corrosion) at the tube sheet weld joint of a shell and tube heat exchanger is typically the result of multiple interacting factors. The main reasons can be summarized into the following four aspects:
- Inherent Deficiencies from Manufacturing and Welding
The welding process itself introduces issues that can become initiation points for corrosion:- Weld Defects: Processes like manual arc welding can produce undercuts, porosity, and slag inclusions. These areas tend to accumulate corrosive media and become the starting points for corrosion.
- Microstructural and Stress Changes: The high heat of welding alters the metallic structure in the weld zone, often reducing its corrosion resistance. Additionally, welding introduces residual stresses that can combine with operational stresses, potentially leading to stress corrosion cracking.
- Chemical and Electrochemical Attack from the Cooling Water Medium
This is the most common and significant cause of rusting, especially when the tube sheet is in contact with industrial cooling water:- Electrochemical Corrosion: Cooling water acts as a natural electrolyte solution, containing dissolved oxygen, chloride ions, salts, and more. A potential difference exists between the weld metal and the base tube sheet metal, forming a corrosion cell that accelerates corrosion at the weld. Specifically, chloride ions are highly penetrative and can break the passive film on the metal surface, leading to pitting and crevice corrosion.
- Direct Chemical Corrosion: If the process medium itself is corrosive, it can directly cause chemical corrosion of the tube sheet and weld.
- Galvanic Corrosion from Dissimilar Metal Contact
When the tube sheet and tubes are made of different materials (e.g., a carbon steel tube sheet with stainless steel or copper tubes), galvanic corrosion can occur in the presence of an electrolyte. In this scenario, the more active carbon steel tube sheet and its weld area act as the anode and are corroded preferentially. - Influence of Operating Conditions and Environment
Several external factors during operation can significantly accelerate the corrosion process:- Temperature: Corrosion is a chemical reaction; generally, the corrosion rate approximately doubles for every 10°C increase in temperature.
- pH Value: The more acidic the medium (lower pH), the more severe the corrosion tends to be.
- Flow Velocity: High-velocity media can cause erosion-corrosion on the weld, removing protective films and accelerating metal loss.
- Impurities and Microorganisms: Solid particles, sulfide ions, ammonia ions in the cooling water, as well as microorganisms, can deposit on the weld, leading to under-deposit corrosion.
Related References:
1. austenitic stainless steel
2. Stress Corrosion Cracking SCC
3. Chloride Stress Corrosion Cracking (CLSCC)
4. Stress Corrosin Cracking SCC of Duplex Stainless Steel
5. Chloride Stress Corrosion Cracking in Austenitic Stainless Steel
6. Recommendations for Assessing Susceptibility to CLSCC
7. Main Findings on CLSCC in the Reactors
8. Literature Review to Chloride Stress Corrosion Cracking
9. CLSCC Chloride Stress Corrosion Cracking Mechanism
10. Factors Affecting CLSCC Chloride Stress Corrosion Cracking
11. Controlling Chloride Stress Corrosion Cracking
12. Structural Integrity Assessment
13. Non-Destructive Examination NDE
Stress Corrosion Cracking SCC of Duplex Stainless Steel
Residual Stress
Metallographic Test – Metallography Testing
Metallographic Test Report
Stress Corrosion Cracking (SCC)
Chloride Stress Corrosion Cracking
Corrosion of Piping
Corrosion Process
Surface Coatings for Corrosion
Corrosion Resistant Material
Bi- Metallic Corrosion.Galvanic Corrosion
Stainless Steel Corrosion
Corrosion of Piping
Corrosion Process
Surface Coatings for Corrosion
Corrosion Resistant Material
Bi- Metallic Corrosion.Galvanic Corrosion
Intergranular Corrosion
Intergranular Corrosion of Stainless Steel Tubes
Corrosion Resistant Stainless Steel Tube
Corrosion Resistance of Stainless Steel Tubes
Seawater Resistance of Stainless Steel Tubes
Corrosion Mechanism in Stainless Steel Tube
ASTM A262 Intergranular Corrosion Test IGC
ASTM E112 Standard Test Methods for Determining Average Grain Size
Methods of minimizing chloride stress corrosion cracking