The qualification of welders and welding procedures often involves macro-examination of welds. Performing macro-etches is a relatively simple procedure, but strict guidelines must be followed to ensure safety. Most chemicals used in etching solutions are hazardous and must be handled accordingly. Any person etching with these chemicals must familiarize him or herself with the corresponding Safety Data Sheet (SDS). Always review the SDS sheets before using or handling any chemical.  If you are unsure always seek help. 

We received many inquiries from our readers regarding marcroetching practices when we first published Qualification of Welding Procedures, Welders and Welding Operators.  This is important as many fabricators perform their own qualifications. The scope of the above-mentioned publication was structural steel, but its principles can be followed for other materials. This article goes beyond just steel and looks at etching for other metals such as aluminum, copper alloys, stainless steel, nickel alloys, and titanium alloys.

Different alloys require different etching solutions. If you work with many alloys, you will need to have several chemicals available. It is imperative that you have Standard Operating Procedures (SOPs) in place that thoroughly explain handling procedures as well as mixing procedures. Do not attempt to mix any of these chemicals without proper training.

The microstructure of welds can be revealed using a variety of etching techniques. The weld metal, the partially melted zone around the weld, and sometimes the heat-affected zone (HAZ) are not homogeneous, so they tend to etch differently than base metals. Etching techniques can be divided into chemical methods, electrolytic methods, and staining methods. In general, chemical methods are simpler to apply and require less equipment, so they tend to be favored by the nonspecialist. Electrolytic methods tend to be favored by those who specialize in the examination of corrosion-resistant alloys. Various staining methods help to bring out phases of interest at higher contrast (or with color) than possible with the other techniques. This article will primarily focus on chemical etching methods, which are widely used due to their simplicity.

Etching of Carbon and Low-Alloy Steels

For plain-carbon and low-alloy steels, several etchants work well to reveal macro-etching details like weld penetration or general weld shape.

• Nitric Acid (Nital) – A dilute mixture of 5 – 10% nitric acid with the balance being alcohol. Water will work, but alcohol is preferred. Use at room temperature by immersing the polished sample until the desired etching effect is achieved. Mixtures of 1-5% are also common for microstructural examination.  This is the most commonly used etching solution for carbon and low alloy steels.
• Ammonium Persulfate – Mix one part ammonium persulfate to nine parts water by weight. Use at room temperature. Apply with a cotton swab.
• Iodide and Potassium Iodide – Mix one part iodide, two parts potassium iodide and ten parts water by weight. Apply with a brush at room temperature.
• For alloys with increased chromium content, such as 2.25Cr–1Mo steel and 9Cr steels, more aggressive reagents like Vilella’s reagent are often used. There are also a number of etchants that include picric acid, which are excellent for revealing fine microstructural details like pearlite colonies and martensite lath structure.

Etching of Stainless Steel

Because stainless steels contain a minimum of 11% chromium, more aggressive etching solutions must be used to reveal the weld microstructure.

• Ferric Chloride/Nitric Acid Mixture – For stainless steels, stick with the following mixture: 200g ferric chloride (FeCl3) + 300mL nitric acid (HNO3) + 100mL water (H2O). Apply at room temperature with a cotton swab or immerse the part in the solution.
• Quick alternative: Printed Circuit (PC) Board etchant containing Ferric Chloride – apply to specimen when warm and flush quickly. PC Board Etchant can be used to etch austenitic stainless steels.
• Mixed Acids and Glyceregia are both general-purpose etchants that work well to reveal microstructure in most stainless steel alloys and also serve well for macroetching.
• Electrolytic Etching: Techniques like 10% oxalic acid are widely used to reveal carbide precipitation at grain boundaries in ferritic and austenitic alloys. It also provides good contrast of ferrite and austenite in weld metals and segregation effects in fully austenitic weld metals. The 40% NaOH electrolytic technique provides good ferrite–austenite phase contrast in duplex stainless steels and is also used to detect sigma-phase precipitation.
• Staining Techniques: To differentiate ferrite from austenite in austenitic and duplex grades, staining techniques have been developed that give good contrast between the phases, sometimes even with color. 
• Dissimilar Metals: For welds between dissimilar metals uch as  austenitic stainless steels and carbon steels, a two-step procedure is recommended. First, etch with nital to attack the carbon steel without etching the stainless steel. Then, electrolytically etch with a 10% chromic acid solution. The prior nital etching protects (passivates) the carbon steel and prevents additional attack by the chromic acid solution.

Etching of Aluminum Alloys

The solution that works for aluminum is also a three-part mixture composed of the following. It should be used at room temperature.

• 15mL hydrochloric acid + 10mL hydrofluoric acid + 85mL water
• Quick alternative: Oven cleaner.  

Etching of Copper Alloys

For copper and copper alloys use concentrated nitric acid. Immerse the specimen in concentrated nitric acid for 30 – 45 seconds. Then rinse with water. After a thorough rinse, immerse in a mixture of 50% nitric acid/50% water. Rinse again and remove the white residue with alcohol or water.

Etching of Nickel Alloys

A solution that works well for nickel alloys is a three-part mixture that should be used fresh every time. The specimen must be immersed in this solution for 1 – 2 minutes. The mix consists of:

• 1 part hydrogen peroxide (H$_2$O$_2$) + 2 parts hydrochloric (HCl) acid + 3 parts water (H$_2$O)

Both chemical and electrolytic methods are useful for nickel-base alloys. Chemical methods are generally simpler and require less equipment.

Etching of Titanium Alloys

For titanium, use the following mixture at room temperature by immersing the specimen or applying the solution by swabbing.

• 3mL hydrofluoric acid + 6mL nitric acid + 90mL water

Fracture Surface Cleaning

When performing fracture analysis, especially on steel samples exposed to the environment, oxide scale or “rust” can obscure the fracture surface. Removing this is crucial for accurate analysis.

• Detergent Cleaning: For surface debris and light oxidation, water-based detergent cleaning can be used. A common detergent is Alconox®, which comes in powder form. Prepare a solution by dissolving 15g of Alconox in approximately 350ml of water, heat to approximately 95°C (205°F), and place in an ultrasonic cleaner. Immerse the sample under ultrasonic agitation for up to 30 minutes, rinse with alcohol, and dry.
• Aggressive Solutions for Heavy Oxidation: For heavily oxidized surfaces, more aggressive solutions are needed. A very effective method for removing tenacious oxide from steel and stainless steel surfaces uses a reagent solution consisting of concentrated hydrochloric acid (HCl) and 10 g/liter of 1,3-n-butyl-2 thiourea (known as DBT). This stock solution is then diluted 50/50 with distilled water prior to use. Immerse the sample in this solution under ultrasonic agitation for a short period until the oxidation is removed. This technique is quite fast (usually only a few minutes of agitation) and has been shown to do virtually no damage to the underlying fracture surface. It works particularly well with carbon and low-alloy steels but is also effective in cleaning stainless steels.

Safety First!

Remember to follow safety rules when handling these solutions. Always examine Safety Data Sheets (SDS) to understand the risks and the remedies should problems occur. Always use these acids and solutions in a well-ventilated area and with quick access to water for a thorough rinse (eye and full body). When not in use these solutions must be capped, stored, and locked in a secure location.

References:

The Procedure Handbook for Arc Welding

Qualification of Welding Procedures, Welders and Welding Operators

AWS B2.1/B2.1M:2014- Specification for Welding Procedure and Performance Qualification

 Welding Metallurgy and Weldability by John C. Lippold