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Stainless Steel Flanges

Failure analysis of acid washing corrosion of stainless steel flanges

Stainless steel flanges are widely used in
ship pipeline engineering because of their good corrosion resistance. As an
important part of pipeline connection, they have the advantages of convenient
connection, easy to maintain pipeline sealing performance, and easy inspection
and replacement of certain pipelines. A certain type of ship has recently
purchased a batch of 304 flange. The flange is sent to the pickling plant for
pickling and passivation treatment before use. The pickling tank is placed in
the pickling tank for ten minutes. Corrosion was observed after cleaning. In
order to find out the cause of corrosion of the batch of flanges, prevent
product quality problems from happening again, and reduce economic losses, we
conducted chemical analysis and metallographic examination of the batch flange
samples.

1 Physical and chemical testing

1.1 Chemical composition analysis

 
Chemical analysis samples were taken on the corrosion flange, and the
chemical composition was determined by a Baird DV-6 type spark direct reading
spectrometer. The results are shown in Table 1. According to the technical
requirements of 304 stainless steel chemical composition in ASTM 276-2013
“Standard Specification for Stainless Steel Bars and Shapes”, the
content of Cr in the chemical composition of the failed flange is lower than
the standard value.


Table I

1.2 Metallographic examination

  A
longitudinal section sample was taken at the corrosion of the failed flange.
After polishing, it was not etched. Under the ZEISS metallographic microscope,
the non-metallic inclusions were measured according to GB/T 10561-2005. Rating
chart microscopic test method: 1.5 grades of sulfides; grade 0 of alumina;
grade 0 of silicates; grades of grades of spherical oxides.

  The
sample was etched by aqueous solution of ferric chloride hydrochloride and
observed under a 100 x metallographic microscope. The austenite grains in the
material were found to be extremely uneven. The grain size grade was determined
according to GB/T6394-2002 “Metal average grain size”. According to
the method, the coarse-grained zone can be rated as 1.5 (see Figure 3); the
fine-grained zone can be rated as 4.0.

 the fine-grained zone can be rated as 4.0

Figure 3

Observing the microstructure of the
near-surface corrosion, it can be found that the corrosion starts from the metal
surface and concentrates on the austenite grain boundary and extends into the
interior of the material. The grain boundary of this region is destroyed by
corrosion and the intergranular bond Almost completely lost strength, corroded.
Severe metals even form powders that are easily scraped off the surface of the
material.

  The
high-magnification structure of the corrosion flange was observed by a 500x
metallographic microscope, and the microstructure was austenite + a small
amount of ferrite + the first phase particles precipitated on the grain
boundary.

2 Comprehensive analysis

  The
results of physical and chemical tests show that the content of Cr in the
chemical composition of the stainless steel flange is slightly lower than the
standard value. The Cr element is the most important element determining the
corrosion resistance of stainless steel. It can react with oxygen to produce
oxides of Cr and form blunt. The layer acts to prevent corrosion. Moreover, the
non-metallic sulfide content in the material is high, and the aggregation of
the sulfide in the local region causes the concentration of the Cr element in
the surrounding area to decrease, forming a Cr-depleted region, thereby
affecting the corrosion resistance of the stainless steel.

 
Observing the grain of the stainless steel flange, it can be found that
the grain size is extremely uneven, and the mixed crystal grains with uneven
size in the structure easily form the difference of the electrode potential,
resulting in micro-battery, which leads to electrochemical corrosion on the
surface of the material.

The coarse-grained mixed grain of the
stainless steel flange is mainly related to the hot working deformation process
due to the sharp deformation of the grain during forging.

 
Analysis of the microstructure of the near-surface corrosion of the
flange shows that the corrosion starts from the surface of the flange and
extends along the austenite grain boundary. The high-magnification
microstructure of the material shows the austenite on the grain boundary. There
are many first phase precipitates, and the third phase accumulated on the grain
boundary is easy to cause the grain boundary to be depleted in chromium,
causing intergranular corrosion tendency and greatly reducing its corrosion resistance.

  The
third phase in stainless steel mainly has fine carbides (M 23C6 ), σ phase and
δ ferrite, all of which have a great influence on the corrosion resistance of
stainless steel. The formation temperature of the precipitated phase of M23C6
is 450 °C-850 °C, mainly composed of metal chromium. Most of the carbides are
distributed on the grain boundaries of the crystals, and some are distributed
inside the crystals and crystal defects because the carbides are rich.
Chromium, easily lead to chromium deficiency in the region; σ phase formation
temperature is 500 ° C -925 ° C, in this temperature zone, ferrite partially or
completely decomposes
σ phase, 6 phase chromium content is 42% -50% It is a brittle phase
with high hardness, which can cause the toughness and corrosion performance of
the material to decrease. δ ferrite is a kind of high-temperature ferrite which
is formed by cooling from liquid iron to 1538 °C. The phase is brittle and
processed. It is easy to cause cracks and is prone to pitting corrosion.

3 Conclusion

 
Through a series of failure analysis of corroded stainless steel
flanges, the following conclusions can be drawn:

  •   (1)
    Corrosion of stainless steel flanges is the result of a combination of factors,
    in which the first phase precipitated on the grain boundaries of the material
    is the main cause of flange failure. It is recommended that the heating
    temperature be strictly controlled during the hot working process, not
    exceeding the upper limit temperature of the material heating process
    specification, and cooling rapidly after solid solution to avoid staying in the
    temperature range of 450 ° C – 925 ° C for a long time to prevent the
    precipitation of the third phase particles.
  •   (2)
    The mixed grains in the material tend to cause electrochemical corrosion on the
    surface of the material, and the forging ratio should be strictly controlled in
    the forging process.
  •   (3)
    The low content of Cr in the material and the high content of sulfide directly
    affect the corrosion resistance of the flange. When selecting materials,
    attention should be paid to the selection of materials with pure metallurgical
    quality.

Source:
China Stainless Steel Flange Manufacturer – Yaang Pipe Industry Co., Limited (
www.yaang.com)

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