What is epoxy coating?

The importance of epoxy coatings for corrosion protection cannot be overstated. The global market for epoxy coatings is predicted to grow at a CAGR of 5.32% to surpass USD 44.61 billion by 2028. It is no wonder that a quick search of the keywords ‘epoxy coating’ brings up more than 88 million results.

This article will go through the definition and main reactions that form the epoxy coating.

What is epoxy coating? Definition and working principle

Epoxy coating is a layer of protective organic polymer coated on a metallic or non-metallic substrate. It is the chemical product formed after a reaction between two components—resin and hardener. It consists of additional components that contribute towards specific properties.

The resin is a polymer that contains the functional group epoxide which is a triangular ring of one oxygen and two carbon atoms.

Figure: Epoxide ring

This epoxide ring may then be connected to a varying number of hydrogen atoms or other C-H groups in a long polymer chain. The backbone chain may be made up of esters, acrylics, ethyl or methyl groups, etc.

The key to recognising an epoxy coating is the epoxide ring in the resin chemical structure.

The hardener may vary depending on the curing properties required for the application. Here’s a quick list of the various hardeners:

  • Amines – aliphatic, aromatic, and cycloaliphatic
  • Anhydrides
  • Phenols
  • Thiols

The reactivity of the hardeners increases in the following order:

Phenols < Anhydrides < Aromatic amines < Cycloaliphatic amines < Aliphatic amines < Thiols

The following images show the typical chemical reaction for an epoxide resin with an amine hardener. The epoxy reaction occurs in three steps:

  1. The epoxide ring in the resin reacts with the amine group in the hardener to form a secondary amine. In this step, one H atom from amine goes and forms OH with the O in the epoxide group leading to the opening up of the ring. The N gets connected with the C in the open epoxide molecule.
  2. Next, the formed secondary amine once again reacts with the epoxide group. Here, the remaining H from the secondary amine breaks the epoxide ring. H goes to the O from the epoxide group. The C from the backbone molecule connects with the N from the amine group. This reaction forms the tertiary amine.
  3. The last step is the etherification of the tertiary amines. The tertiary amine reacts with one more epoxide ring. this step is different from the previous steps because the N in the amine group does not participate in this reaction. The Hydroxyl OH groups are the active sites. The H from the OH group of the tertiary amine breaks the epoxide ring and reacts with the O to form OH. Thus, it shifts from OH in tertiary amine to OH in the epoxide group. The remaining O in the tertiary amine now forms a crosslinking bond with the C of the backbone CH2 group of the epoxide ring. This is the final epoxy coating molecule.
Figure: Reaction scheme of the epoxy resin and amine hardener to form epoxy coating.

Wrap up

Epoxy coating is a popular coating. However, its reaction mechanism is often misunderstood or not understood at all. This series of articles will cover the principle, reactions, and failures of epoxy coatings.

This article dealt with the amine + epoxy reaction. In the next part of the article, we will deal with the reaction mechanisms of other hardeners used to form epoxy coatings.

Corrosion of concrete electric poles

Concrete electric poles have been used extensively for supporting heavy transmission lines for several years. In this article, we will cover the types of concrete poles, their requirements, their corrosion, and their inspection techniques.

Source

Types of concrete poles

There are two types of concrete poles:

  • PCC: PCC stands for Plain Cement Concrete. This type of PCC pole has an entire concrete structure without any metallic reinforcement. The preparation comprises of inserting a high-tensile steel wire in a mould and stretching it to a predetermined extent. Another wire, which is galvanised steel wire, is introduced as the earth wire in the mold. The mould is then filled with concrete mix, which is then compressed. Such poles are also called pre-stressed cement concrete poles (PSC) as the compression leads to a stress generation in the concrete mix.
  • RCC: RCC is short for Reinforced cement concrete. As the name suggests, this type of concrete pole is reinforced with TMT steel bars for greater strength and endurance. In the production of RCC poles, a structure of TMT bars is first assembled and then embedded in pol-shaped concrete slabs.

Specifications

Specifications for PSC electric poles

  • Overall length of pole
  • Working load
  • Bottom depth
  • Top depth
  • Breadth
  • Depth of planting
  • Factor of safety
  • Diameter of HT pre-stressing wire
  • Minimum tensile strength of HT pre-stressing wire
  • Number of HT wire
  • Concrete grade
  • Ultimate load
  • Cube crushing strength of concrete at 28 Days
  • Allowable max compressive strength in concrete at working load

Specifications for RCC electric poles

  • Overall length of poles
  • Cement grade and composition
  • Composition and grade of aggregates
  • Mechanical properties of reinforcing bars and wires
  • Surface condition and preparation of reinforcing bars
  • Composition of admixture
  • Quality and composition of water used for concrete preparation
  • Maximum wind pressure
  • Depth of planting
  • Transverse strength at failure
  • Welding and lapping of reinforcement
  • Earthing wire installation

Corrosion of PSC and RCC poles

  • Spalling of concrete cover
  • Uniform or localized corrosion of metallic wires and bars due to ingress of moisture and oxygen through the concrete pores
  • Soil corrosion and concrete degradation in the areas buried under the earth due to ingress of chlorides, sulphates, nitrates, and moisture from the soil.
  • Saturation of concrete at the soil/atmosphere interface due to capillary absorption of groundwater due to wicking effect
  • Wind erosion of the concrete at greater heights.
  • Crevice corrosion of wires and poles at the inlet and outlet points of wires inserted in the poles.
  • Uniform corrosion of bare wires
  • Pitting corrosion at areas left bare due to insufficient or damaged coating
  • Stress corrosion cracking of the reinforcing bars or embedded wires due to local stress concentrations in the concrete

Inspection and testing

Here are a few typical inspection measures and tests conducted to assess the integrity of the concrete electric poles:

Wrap up

Concrete poles have been used extensively for transmission lines and utilities. While the basic specifications are common, there may be variations in certain parameters with respect to the location and climate.

Hence, a comprehensive view of the requirements and testing conducted globally will help assess the corrosion causes and select the most suitable inspection measures to nip the corrosion in the bud.

References

  • https://www.cracindia.in/admin/uploads/IS-785.pdf
  • https://www.wbsedcl.in/irj/go/km/docs/internet/webpage/techspec/Technical_Spec_of_Pole.pdf
  • https://www.materialsperformance.com/articles/material-selection-design/2019/01/control-of-environmental-degradation-of-concrete-power-poles
  • https://www.materialsperformance.com/articles/corrosion-basics/2020/11/bend-test-of-concrete-power-poles
  • https://www.irjet.net/archives/V8/i5/IRJET-V8I5749.pdf

Corrosion of electric poles

What are electric poles?

Electric poles are long hollow tubular poles that host
overhead electricity distribution wires. They may also have equipment to
monitor and regulate the electric supply.

What are the types of electric poles?

There are two types of electric poles:

  1. Swaged  
  2. Stepped

What are the materials used for making electric poles?

  • Mild steel is the material of choice for electric poles.  
  • They are longitudinally welded tube sections of hot-rolled structural carbon steel.
  • There also has to be an MS base plate welded to the bottom of the pole.

The standards typically used to select the grade of mild steel are:

  • JIS G 3444 ( ST-51 )
  • DIN 17100 (ST-52)
  • BS-4360
  • IS: 2713 (Part – I, II): 1980 

Chief mechanical properties required 

  • Tensile strength
  • Yield strength

The specifications for the poles include the following
parameters:

  • Maximum Ambient temperature
  • Minimum ambient temperature
  • Maximum relative humidity 
  • Average number of thunderstorm day per annum
  • Maximum no. of rainy days/annum
  • Average Rainfall
  • Maximum Wind pressure/wind speed
  • Height above sea level (m) not exceeding
  • Earthquake acceleration horizontal seismic co-efficient

Corrosion-prone areas of electric poles

  1. The weld area of the bottom plate may undergo galvanic or uniform corrosion. It may also experience weld decay due to chlorides in the soil.
  2. The surface of poles is susceptible to uniform corrosion.
  3. Microbial corrosion of area submerged in soil.
  4. Crevice corrosion is possible at overlaps and fixtures.
  5. Pitting corrosion at areas where water can accumulate, such as nooks and corners in the design.
  6. Erosion corrosion of upper areas of the poles may occur due to dust carried through the wind.
  7. Coating delamination and uniform corrosion underneath is possible due to moisture absorption.

Coatings for electric poles

The coatings recommended for electric poles consist of the layers of the following components:

  • Galvanization layer
  • Red oxide paint
  • Black bituminous paint from the bottom of the pole up to the region buried in the soil
  • Long-oil alkyd with aluminium, zinc, or stainless steel dust
  • Epoxy coating as sealer

References