Soil Side Corrosion of the Bottom of Caustic Soda Tank T35201A Failure Analysis Report

Prepared By : Ahmed ReyadHp

04/12/2013

Soil Side Corrosion of the Bottom of Caustic Soda Tank T35201A Failure Analysis Report

IntroductionCaustic soda tank T35201A is an above ground storage tank used for the storage of 50% caustic soda. The tank is made from carbon steel with internal rubber lining for the purpose of product quality.The tank was handed over for complete inspection, internally and externally.Visual inspection revealed bulges in the rubber lining. Some rubber pieces had been removed for inspection found the metal was completely corroded.Other non-destructive testing, SLOFEC technique, had been used for complete scanning of the tank bottom where severe corrosion was detected on the tank periphery. After completion of inspection, temporary repair of the corroded areas was carried out.This report will study the causes of this severe corrosion and recommend the proper measures to avoid and eliminate repeating this case in the future.

Contents - Design data of T35201 A- Inspection observations- Discussion.- Conclusion- Recommendations.- References

Design data of T35201 A Diameter 50 m Height 15 m

Design Temperature 65oC Design Pressure Atmospheric

Stored Product Caustic Soda 50 % Specific gravity 1.51

Bottom material A36 Bottom thickness 6.4 mm

Inspection observations Inspection of the tank bottom will be divided into three stages

1. Inspection before SLOFEC Scanning :a. External Visual Inspection: Rubber sealant between the chime (bottom projection) and the concrete

ring wall was cracked and highly deteriorated and concrete grout was cracked at many locations along the periphery of the tank.

b. Internal Visual Inspectioni. Bottom: Blistering and depressed areas in the lining of the tank bottom were observed.ii. Drain sumps: lining of the shell of the drain sumps seemed to be in a very bad condition

where blistering and highly depressed areas were observed. By removing the rubber in this areas ,completely perforated areas were observed

2. Bottom scanning examination (SLOFEC technique):

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Soil Side Corrosion of the Bottom of Caustic Soda Tank T35201A Failure Analysis Report

- Bottom plates were completely scanned by SLOFEC technique (Saturation Low Frequency Eddy Current) to

determine corrosion, pitting, and wall thickness loss.

- Before starting the scanning of the bottom plates, the scanner was rolled over a calibration plate of similar

material and thickness having artificial defects. The bottom plates were divided into several tracks.

a. The governed corrosion mechanism is Soil –side corrosionb. The area showing above 50% wall loss (by SLOFEC Scanning) is confined within a 1: 2 m area of the

junction of the shell to bottom plate, along the periphery of the tank.c. Highly corroded areas were concentrated in the

3. Inspection after SLOFEC Scanning:a. Internal Visual Inspection & Ultrasonic Thickness Measurements

- By removing the rubber in the areas showing more than 50% metal loss by SLOFEC scanning, completely perforated areas is visually observed, and thickness reduction (more than 50% metal loss) is detected by ultrasonic measurements.

Discussion

Corrosion mechanism:- The bottom of T35201 A is found highly corroded at the periphery of the tank.

- The inspection results confirmed that the governing mechanism is Soil Side Corrosion.

- the following is the contributing factors which causing this mechanism :

1- Sealant deterioration: will cause water ingress under the tank bottom.

2- Tank loading and unloading will cause voids formation

3- Humid atmosphere.

ConclusionThe bottom of T35201-A was found highly corroded due to soil side corrosion.

Recommendations- Protection against soil side corrosion is highly recommended using Vapor Corrosion inhibitors (VCI).- Proper Sealant system should be applied to prevent water ingress under tank bottom, and to be frequently

inspected and maintained.

References

- API 653 ,inspection of above ground storage tanks- Zerust ,vapor corrosion inhibitor

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