Eddy Current Coating Thickness Gauges (ECT)

Dear Readers,

In continuation with my earlier post of DFT, let us know the ECT (Eddy Current Thickness)

As you may aware that Dry film thickness (DFT) or coating thickness is arguably the single most important measurement made during the application and inspection of protective coatings. Dry film thickness can be measured on either magnetic steel surfaces or non-magnetic metal surfaces such as stainless steel or aluminum using a digital coating thickness gauge.

The eddy current principle is used for non-conductive coatings on non-ferrous metals substrates. They can be divided into two types: contact and non-contact. The contact type measures coating thickness by the amplitude of eddy currents, while the non-contact type measures coating thickness by the phase difference in eddy currents

Eddy current dry film thickness gauges are used in eddy current testing (ECT), a non-destructive test (NDT) method that determines dry film thickness based on the strength or level of eddy currents induced in a metal by a probe containing a conducting wire coil. The distance between the probe and the substrate, which is equivalent to the coating thickness, inversely affects the eddy current strength level. So, the strength of the eddy current deceases as the coating thickness increases.

An eddy current dry film thickness gauge is a widely used non-destructive coating thickness measurement tool because of its high sensitivity and wide range of thickness capability.

For More Details about Dry Film Thickness (DFT) click Here

Dry film thickness

Dear Readers,

Over the past several decades, industrial painting has changed, owing to the use of new, more durable finishes. Coating is the most critical component in an anticorrosive coating system. The main function of coating is to provide corrosion protection by adhesion to the substrate. A coating thickness gauge (also referred to as a paint meter) is used to measure dry film thickness.

Dry film thickness (DFT) is the thickness of a coating as measured above the substrate. This can consist of a single layer or multiple layers. DFT is measured for cured coatings (after the coating dries). Thickness of a coating depends on the application and type of process employed. DFT is a parameter that is considered during quality control or inspection processes.

Dry film thickness (DFT) can be measured using two methods: destructive thickness measurement, where the coating is cut to the substrate using a cutter; and non-destructive coating thickness measurement, using techniques which do not damage the coating

Dry film thickness can be measured on either magnetic steel surfaces or non-magnetic metal surfaces such as stainless steel or aluminium using a digital coating thickness gauge. The principle of electromagnetic induction is used for non-magnetic coatings on magnetic substrates such as steel. The eddy current principle is used for non-conductive coatings on non-ferrous metals substrates.

Types of DFT gauges include:

Magnetic pull-off (Type I)

Constant pressure probe (Type II)

Electronic magnetic flux

Eddy current gauge

Standards concerning DFT measurements include:

ISO 2808

BS 3900 Part C5

BS 5494

ASTM D1005

D1400

D2691

SIS 184160

Surface Preparation- SSPC Standard

Dear Readers,

SSPC is a short form of Steel Structure Painting Council.

SSPC: The Society for Protective Coatings was founded in 1950 as the Steel Structures Painting Council, a non-profit professional society concerned with the use of coatings to protect industrial steel structures. SSPC creates, publishes and maintain standards for coatings that protect and preserve concrete, steel and other industrial and marine structures and surfaces. These standards address surface preparation, coating selection, coating application, environmental regulations and industry health and safety issues to the coating industry.
 

SP1 - Solvent Cleaning: The method of solvent cleaning is intended to remove “all visible oil, grease, soil, drawing and cutting compounds, and all other soluble contaminants from steel surfaces.” by cleaning with solvent, vapour, alkali, emulsion or steam.

SP2 - Hand Tools Cleaning: This standard covers the removal of loose mill scale, loose rust, loose paint, and other loose detrimental foreign matter from surfaces with the use of non-powered hand tools. Cleaning by chipping, scraping, sanding and wire brushing to a specific degree. Equivalent to ISO St2 Grade.

SP3 - Power tool cleaning: A mechanical method of surface preparation widely used in industry and involving the use of power sanders or wire brushes, power chipping hammers, abrasive grinding wheels, needle guns etc. Although usually more effective than hand tool cleaning, it is not considered adequate for use under severe exposure conditions or for immersion applications. Equivalent to ISO St3 Grade.

SP4 - Flame Clean: Removal of all loose scale, rust and other detrimental foreign matter by passing high temperature, high velocity oxy-acetylene flames over the entire surface, followed by wire brushing. Surface should also be free of oil, grease, dirt, soil, salts and other contaminants.

SP5 - White Metal Blast Cleaning: The removal of all visible rust, mill scale, paint and contaminants, leaving the metal uniformly white or grey in appearance. This is the ultimate in blast cleaning. Use where maximum performance of protective coatings is necessary due to exceptionally severe conditions such as constant immersion in water or liquid chemicals. Equivalent to ISO Sa3 Grade and NACE #1 Definition.

SP6 - Commercial Blast Cleaning: All oil, grease, dirt, rust scale and foreign matter are completely removed from the surface and all rust, mill scale and old paint are completely removed by abrasive blasting except for slight shadows, streaks or discolorations caused by rust stain, mill scale oxides or slight, tight resides of paint or coating that remain.

 If the surface is pitted, slight residue of rust or paint may be found in the bottom of pits; at least two-thirds of each square inch of surface area shall be free of all visible residues and the remainder shall be limited to the light residues mentioned above. Equivalent to ISO Sa2 Grade and NACE #3 Definition.

SP7 - Brush Off Blast Cleaning: A method in which all oil, grease, dirt, rust scale, loose mill scale, loose rust and loose paint or coatings are removed completely. Tight mill scale and tightly-adhered rust, paint and coatings are permitted to remain. However all mill scale and rust must have been exposed to the abrasive blast pattern sufficiently to expose numerous flecks of the underlying metal fairly uniformly distributed over the entire surface. Equivalent to ISO Sa1 Grade and NACE #4 Definition.

SP8 – Pickling: Complete removal of rust and mill scale by acid pickling, duplex pickling or electrolytic pickling.

SP9 - Weather and Blast: Weathering to remove all or part of the mill scale followed by one of the blast cleaning standards.

SP10 - Near White Blast Cleaning: In this method, all oil, grease, dirt, mill scale, rust, corrosion products, oxides, paint or other foreign matter have been completely removed from the surface by abrasive blasting, except for very light shadows, very slight streaks or slight discolorations caused by rust stain, mill scale oxides or slight, tight residues of paint or coating. Blast cleaning nearly to “White Metal” cleanliness, until at least 95% of the surface area is free of all visible residues. Equivalent to ISO Sa2.5 Grade and NACE #2.

SP11 - Power Tool Cleaning to Bare Metal: Utilizing same equipment as Power Tool Cleaning to remove all visible coatings and contaminants to bare metal substrate. It will be applicable where abrasive blasting is not feasible or permissible.

SP12 - Surface Preparation and Cleaning of Metals by Water jetting Prior to Recoating: Water jetting is the use of standard jetting from a nozzle at high pressures (10,000 psi or higher) to prepare a surface for coating. Intended primarily for carbon steel, but applicable on other metals, water jetting does not create a profile on the metal substrate. However, metals with an existing profile acceptable for painting can be cleaned with water jetting to expose the existing profile. Water jetting will remove all loose rust, loose mill scale, and loose coatings uniformly.

SP13 - Surface Preparation of Concrete: Describes requirements for mechanical, chemical and thermal methods of concrete surface preparation to improve the adhesion and longevity of coating systems. This standard applies to all types of cementitious surfaces including cast-in-place concrete floors and walls, precast slabs, masonry walls, and shotcrete surfaces.

SP14 - Industrial Blast Cleaning

SP15 - Commercial Grade Power-Tool Cleaning

SP16 - Brush-Off Blast Cleaning of Coated and Uncoated Galvanized Steel, Stainless Steels, and Non-Ferrous Metals.

Sand Blasting

 

Dear Readers,

 

Sand blasting is one of the surface cleaning process for structural steel. Sandblasting is a general term used to describe the act of propelling very fine bits of material at high-velocity to clean a surface. Sand used to be the most commonly used material.

As per ISO and Swedish standard, Surface preparation by blast cleaning is designated by the letters "Sa". Swedish standards has been accepted worldwide. As per standards Sand blasting is classified under four category namely Sa1, Sa2, Sa 2 ½, & Sa3.


Brief descriptions are as follows. 

Sa1 = Light Blast Cleaning

Sa2 = Through Blast Cleaning

Sa 2 ½ = Very through blast cleaning

Sa3 = Blast cleaning to visually clean Steel

“Sa” means Swedish air blast similarly “St” means Swedish tool cleaning. Detailed description of each grade is as follows.

Blasting Grade Sa 1: It is a light blast cleaning. Loose mill scale, rust and foreign matter must be removed. When viewed without magnification, the surface shall be free from visible oil, grease and dirt, and from poorly adhering mill scale, rust, paint coatings and foreign matter.

Blasting Grade Sa 2: It is a through blast cleaning. Almost all mill scale, rust and foreign matter must be removed.  When viewed without magnification, the surface shall be free from visible oil, grease and dirt, and from poorly adhering mill scale, rust, paint coatings and foreign matter. Any residual contamination shall be firmly adhering.

Blasting Grade Sa 2 ½: It is a very through blast cleaning. This grade is very often used in construction industry. Mill scale, rust and foreign matter must be removed to the extent that the only traces remaining are slight stains in the form of spots or stripes.

Blasting Grade Sa3: Blast cleaning to pure metal. Mill scale, rust and foreign matter must be removed completely and it shall have a uniform metallic colour.

Notes.

1). Surface cleaning should be mention along with rust grade. For example B Sa2, here first letter "B" represents rust grade.

2). Prior to blast –cleaning, any heavy layers of rust shall be removed by chipping. Visible oil, grease and dirt shall also be removed.

3). Finally the surface is cleaned with a vacuum cleaner, clean dry compressed air, or a clean dry brush before painting.

For More Details about surface preparation of structural steel click Here
For More Details about grades of rust click Here
 

Grades of Rust for Steel Structure

Dear Readers,

 

Rust is having important role to finalaise the surface preparation for steel structure before painting. Four Rust grades, designated A, B, C and D respectively, are specified. The rust grades  are defined as follows

 

Grade-A : It is mainly adhering mill scale, but little rust

Grade-B : It is rusting and flaking of mill scale has begun

Grade-C : The mill scale is rushing away, with slight pitting

Grade-D : The mill scale has rusted away, with general pitting

 

Rust Grade photographs are as follows

GRADE-A

 

Steel Surface largely covered with adhering mill scale but little, if any, rust.

 

 

  

 

 

 

 

 

GRADE-B

 

Steel Surface which has begun to rust and form which the mill scale has begun to flake. 

 

 

 

 

 

GRADE-C

 

Steel Surface on which the mill scale has rusted away or from which it can be scraped, but with slight pitting visible under normal vision.

 

 

 

GRADE-D

Steel Surface on which the mill scale has rusted away and on which general pitting is visible under normal vision.

 

 

 

For More Details about Surface Preparation of Structural Steel Click Here

 

Surface Preparation for Structural Steel

Dear Readers,

Effective life of coating of anti-corrosive paint applied to a steel surface is to a very large extent dependent on how thoroughly the surface has been prepared prior to painting.

 

Removing contamination and increasing surface roughness will allow maximum bond strength and adhesion to develop between coating and substrate. If the coating is applied to a well prepared substrate, they will have strong adhesion to that substrate and may be able to withstand these condition.

 

Coating performance is directly related to the quality of surface preparation.

 

Method of Surface Preparation 

1.       High Pressure Fresh Water Washing

2.       Solvent Cleaning or Degreasing

3.       Manual methods of Surface Preparation

4.       Power tool Methods of Surface Preparation

5.       Dry Abrasive Blasting

6.       Rotary or Wheelabrator blasting

7.       Slurry Blasting

8.       Hydro blasting

Blasting Standards

1.       ISO Standard (8501-1)

2.       Swedish Standards ( SIS 05 59 00)

3.       Steel Structure Painting Council Standards (SSPC-SP)

4.       British Standards Institution (BS4232)

5.       NACE Standards (National Association of Corrosion Engineers)

6.       JSRA Standards (Shipbuilding Research Association of Japan)

7.       Major Local Shipyard Standards