Case Study
Case Study
Case Study
Abstract
From the introduction of its first-generation, duplex (austeno-ferritic) steel
has seen a steady increase in popularity [1]. Recently, the production of high-
strength, corrosion resistant super-duplex coil has been implemented in several
applications.
The case study I present in this paper concerns its use in a set of tubes of a
heat exchanger affected by corrosion. The tubes material, S31803 Duplex
stainless steel, is particularly suited to applications where corrosion is likely due
to chlorides and/or sulphides being present. The heat exchanger operates in a
chloride environment due to marine water on the inner side while the outer side
is in contact with a hydrocarbon mixture rich in Sulphur. It also has high resistance
to general, pitting, crevice and erosion corrosion and to corrosion fatigue.
The aim of our study is to understand the corrosion mechanism occurring and
the factors responsible (e.g. the presence of chlorides, sulphides etc.) using
several techniques for the experimental characterization of the materials
including optical microscopy and SEM (scanning electron micrography).
Keywords: Duplex steel, pitting corrosion, supplied cracking, SEM.d
1. Introduction
The first-generation Duplex stainless steels were developed more than 70
years ago in Sweden for use in the sulfite paper industry. Duplex alloys were
originally created to combat corrosion problems caused by chloride-bearing
cooling waters and other aggressive chemical process fluids such as sulphides
depending on the concentrations. Called Duplex because of its mixed
microstructure with about equal proportions of ferrite and austenite, Duplex
stainless steels are a family of grades, which range in corrosion performance
depending on their alloy content. The chemical composition based on high
contents of chromium, nickel and molybdenum improves intergranular and
pitting corrosion resistance. Additions of nitrogen promote structural hardening
by interstitial solid solution mechanism, which raises the yield strength and
ultimate strength values without impairing toughness. Moreover, the two-phase
microstructure guarantees higher resistance to pitting and stress corrosion
cracking in comparison with conventional stainless steels.
Duplex stainless steels have been used in oil refineries around the world due
to their interesting combination of high corrosion resistance, good mechanical
properties and cost effectiveness. Heat exchangers are typical pieces of
equipment where corrosiveness of the process media or cooling water have led
to premature failures on lower alloyed carbon steel or Cr-Mo grades, and where
duplex stainless steel can offer a cost effective solution in many cases.
The nominal chemical composition of three representative duplex stainless
steels is given in table 1.
Table 1: Nominal chemical composition of three typical duplex stainless
steels, wt%.
2. Case study
An Italian refinery decided to use a S31803 duplex as construction material
of a seawater cooled heat exchanger located in the vacuum unit of the process, in
contact with a Sulphur-rich fluid. Service conditions are reported below:
Service conditions:
Tube side: Seawater
T: 30-34°C P: 5kg/cm2
Shell side: Hydrocarbons + 8%H2 +29%H2S
T: 34-43°C P: 3kg/cm2
With S31803, seawater should always be used on the tube-side, with the
tube to-tube-sheet joints seal welded in order to avoid risk for crevice
corrosion between tube and tube-sheet. After few days, the unit presented
bad thermal exchange and it was supposed the presence of corrosion. After
a preliminary treatment with aqueous vapour of the tubes it was possible to
individuate, through hydraulic pressing, the presence of corrosion as
reported in figure 1. The experimental investigation allowed to understand
the cause of corrosion (e.g. the chlorine or the Sulphur-rich fluid) and the
possible mechanism.
3. Experimental analysis
3.1 Pitting analysis
A. Localized damage to, or poor application of, a protective coating and protective
oxide film.
Figure 1: Photograph of the tube section showing pitting corrosion on the external
side.
Figure 2: Optical micrography of the pitting corrosion under study.
The aim of our study is to understand the effective causes of the pitting
corrosion analyzed and to give an interpretation of the possible mechanism
Figure 5: Mechanism of the breakdown of the passive film induced by enrichment of sulphur
at the metal-passive film interface.