Forensic Engineering
Intergranular Corrosion

Intergranular Corrosion

Intergranular Corrosion

Background

Austenitic Stainless Steels present corrosion resistance ability due to high chromium (Cr) content. Chromium naturally forms a thin, adherent and very protective passive oxide layer when exposed into corrosive aqueous environment which is primarily the reason of its high resistance capabilities.

When austenitic stainless is exposed to a corrosive aqueous environment, chromium oxide enriches at the metal interface due to the formation of passive and resistent corrosion layer.

Problem

However, if the material is heated into a range of 510-790ºC, chromium combines with carbon to form Cr-Carbide (Cr23C6) which may precipitates at the grain boundaries and create a general disorder in the main structural geometry of the base material.

As a result, the steel might present microfissures and intergranular channels of material detachments which will make the tube to be prone to suffer several forms of corrosion.

Therefore the material is said to be “sensitized”. Sensitization is attributed due the alloy element composition, degree of temperature the material reaches and the time of heat exposure.

In the photo it is shown a case of intergranular selective attack where the corrosion phenomenon advances toward the inside of the tube and progressing along the grain of the metal.

As a result, the grain loses its alloying capabilities and becomes anode with respect surrounding grains.

Intergranular Corrosion
Engineering
Mollier

Superheated Transition Zone

Heat Transition Zone

Introduction

As superheated steam enters in the transition zone of the turbine and expands into LP section, steam may cross the saturation line, in the Mollier diagram, to eventually form nucleation of moisture drops on blades and disk surfaces.

Consequently, a number of physical-chemical processes will lead to mechanical corrosion-related damage: precipitation of chemical compounds, deposition, acidification, evaporation, drying of surfaces, erosion, stress corrosion cracking, fatigue, chemical transport, flow-accelerated corrosion and molecular diffusion.

Solution

You cannot dodge this fact but you can always mitigate the consequences by enhancing steam purity and chemistry conditioning of the cycle

1.  Improve carryovers control
2.  Lessen boiler level fluctuations
3.  Minimize feedwater injection for attemperation
4. Implement a dedicated and exclusive water treatment program
5.  Enhance impurities monitoring and control
6.  Preservations are critical during shutdowns

Figure> Mollier diagram with three typical turbine cycles A) reheat turbine in fossil fired plant, B) backpressure turbine, C) reheat turbine in a nuclear LWR plant Cortesy of IAPWS
Mollier
Commissioning

Chemical Cleaning

Chemical Cleaning

Description

Chemical cleaning assignments must be evaluated with regard to the technical efficacy and economical cost. Technical efficacy must consider a deep understanding of the chemistry involved in the process and the nature of the deposit constituent to effectively determine the efficiency and selectivity of the solvents during the removal of the impurity.

In addition, another technical important aspect is the compatibility of the solvents under consideration with the cycle materials they will come in contact with in the course of the operation. Having said that, assessment of the cost of cleaning must observe three factors of major influence toward which improvements are directed in Chemical Boiler

1. Cost of materials, equipment and labor required to perform cleaning.
2. Steam Gen downtime and tight schedules
3. Management of wastes produced during the cleaning

In Chemical Boiler we have developed a chemical treatment process where no temporary system is required, the operation can be executed online during the first startup or operational stage and the waste generated is completely environmentally friendly. Contact us to know how ! The following picture depicts the final result of an online chemical cleaning of a industrial steam boiler

Posts navigation

1 2 3 4