Back to Low Carbon Steel
ML00184 - Apr.2023

Dissimilar Steels in a Threaded Connection

The threads of the carbon steel valve body is corroded. The offshore ambient is wet and chloride rich, creating an aggressive environment for the carbon steel threads.

5

99
The threads of the body is corroded. The ambient is wet and chloride rich, creating an aggressive environment for the threads.
M

Material

Low Carbon Steel
  • ASTM A 350 LF2
Ds

Description

The vent was installed in the body in an installation during about 5 years before the was detected. The metallurgical analysis showed the localized dissolution of the carbon steel thread by a galvanic couple to the nobler material in a metal-to-metal contact that remained wet, and chloride contaminated. The corroded threads no longer sustained the internal pressure.

C

Chemistry (% in weight)

Flange Chemistry

0.18C; 0.29Si; 1.00Mn; 0.016P; 0.021S; 0.04Cr; 0.07Ni; 0.01Mo; 0.05Cu.

Associated Images
The location of the threaded fitting started leaking after less than five years in service. Although the corrosion rate was not high, the localized material loss in the threaded surface of the carbon steel flange allowed the connection leakage.
769
VIEW
ML00184 / Apr.2023
Dissimilar Steels in a Threaded Connection
M - Low Carbon SteelP - ForgingAp - OffshoreCd - Localized Dissolution
Cm - Aspect Before AnalysisDf - General View
The painting of the valve body was detaching at the vent location where the fitting was installed, not fully tighted. Three threaded fillets were outside of the body.
190
VIEW
ML00184 / Apr.2023
Dissimilar Steels in a Threaded Connection
M - Low Carbon SteelP - ForgingAp - OffshoreCd - Localized Dissolution
Cm - Aspect After CleaningDf - Painting Detachment
The austenitic stainless steel fitting was removed from the valve body and presented the sdhesion of oily deposits.
150
VIEW
ML00184 / Apr.2023
Dissimilar Steels in a Threaded Connection
M - Low Carbon SteelP - ForgingAp - OffshoreCd - Localized Dissolution
Cm - Aspect Before AnalysisDf - Surface Deposits
The cleaned fitting is sound. No corrosion, only surface stains.
121
VIEW
ML00184 / Apr.2023
Dissimilar Steels in a Threaded Connection
M - Low Carbon SteelP - ForgingAp - OffshoreCd - Localized Dissolution
Cm - Aspect After CleaningDf - General View
The threads of the carbon steel valve body is corroded. The offshore ambient is wet and chloride rich, creating an aggressive environment for the carbon steel threads.
99
VIEW
ML00184 / Apr.2023
Dissimilar Steels in a Threaded Connection
M - Low Carbon SteelP - ForgingAp - OffshoreCd - Localized Dissolution
Cm - Surface AspectDf - Corrosion
Not only the threads were corroded but all regions under the painting that was detached from the surface.
86
VIEW
ML00184 / Apr.2023
Dissimilar Steels in a Threaded Connection
M - Low Carbon SteelP - ForgingAp - OffshoreCd - Localized Dissolution
Cm - Surface AspectDf - Corrosion
Macrosection of the valve body at the thread, showing the position where the fitting was installed.
84
VIEW
ML00184 / Apr.2023
Dissimilar Steels in a Threaded Connection
M - Low Carbon SteelP - ForgingAp - OffshoreCd - Localized Dissolution
Cm - Transverse Section AspectDf - Corrosion
Macrosection detail of the valve body at the thread, showing the metal to metal contact between the carbon steel female thread and the stainless steel male thread.
80
VIEW
ML00184 / Apr.2023
Dissimilar Steels in a Threaded Connection
M - Low Carbon SteelP - ForgingAp - OffshoreCd - Localized Dissolution
Cm - Transverse Section AspectDf - Localized Corrosion
Micrography of the thread at the valve body surface, showing localized dissolution of the thread.
73
VIEW
ML00184 / Apr.2023
Dissimilar Steels in a Threaded Connection
M - Low Carbon SteelP - ForgingAp - OffshoreCd - Localized Dissolution
Cm - Nital Etched Micrography by OMDf - Corroded Thread
Micrography of the valve body carbon steel thread, in the valley. Ferritic-bainitic structure.
77
VIEW
ML00184 / Apr.2023
Dissimilar Steels in a Threaded Connection
M - Low Carbon SteelP - ForgingAp - OffshoreCd - Localized Dissolution
Cm - Nital Etched Micrography by OMDf - Transverse Section
Micrographic detail of the valve body carbon steel thread, in the valley. Ferritic-bainitic structure.
69
VIEW
ML00184 / Apr.2023
Dissimilar Steels in a Threaded Connection
M - Low Carbon SteelP - ForgingAp - OffshoreCd - Localized Dissolution
Cm - Nital Etched Micrography by OMDf - Bainitic Ferrite
Micrography of the valve body carbon steel thread, in the peak. Ferritic-bainitic structure.
64
VIEW
ML00184 / Apr.2023
Dissimilar Steels in a Threaded Connection
M - Low Carbon SteelP - ForgingAp - OffshoreCd - Localized Dissolution
Cm - Nital Etched Micrography by OMDf - Corroded Thread
Micrographic detail of the valve body carbon steel thread, in the peak. Ferritic-bainitic structure.
55
VIEW
ML00184 / Apr.2023
Dissimilar Steels in a Threaded Connection
M - Low Carbon SteelP - ForgingAp - OffshoreCd - Localized Dissolution
Cm - Nital Etched Micrography by OMDf - Corroded Thread
Cross-section near a threaded surface showing an acicular matrix structure bordered by a dark, layered corrosion/oxide band along the thread root.
1
VIEW
ML00184 / Apr.2023
Dissimilar Steels in a Threaded Connection
M - Low Carbon SteelP - ForgingAp - OffshoreCd - Localized Dissolution
Cm - Nital Etched Micrography by OMDf - Oxide Layer