Stainless Steel - Surface Finishing

Topics Covered

Introduction
Passivation
Pickling
     Pickling Paste
Degreasing
Electropolishing
Grinding and Polishing
Mechanical Cleaning
     Sand Blasting
     Wire Brushing
     Barrel Finishing and Vibratory Finishing
Blackening

Introduction

Stainless steels are used in a wide variety of applications, due to their excellent corrosion resistance properties. In order to achieve this it is necessary to carry out appropriate cleaning and finishing processes during fabrication, thereby preventing any impairment in steel surfaces.

Passivation

Passivation is carried out to eliminate iron from the surfaces of stainless steels by developing a passive surface layer. The presence of carbon steel, low alloy steel, mild steel, cast iron or any iron particles on the surfaces will tend to develop pitting corrosion of the cells between the stainless steel and free iron. This major problem occurs because of the contamination resulting from grinding swarf, or scraping carbon steel fixtures or tools.

The table below provides the procedures carried out to remove iron from the surfaces:

Table 1. Passivation procedures covered under ASTM A380

A trial treatment should be carried out to prevent dulling of the metal surfaces. Thorough rinsing needs to be carried out after all passivation treatments. All precautions necessary for handling acids should be carried out – nitric acid is strongly acidic and harmful to skin.

Pickling

Pickling is a process of reducing the high temperatures of stainless steels, from hot working, heat treatment or welding, by means of acid treatment. This process also aids in removing steel or iron particles and red rust from the corroded steel surfaces. Corrosion particles formed after rusting of free iron cannot be removed using passivation. Besides producing undesirable aesthetics, rusting of surfaces due to high temperatures also minimizes the corrosion resistance of the steel surface layer.

The type of corrosion scale and its removal method pertain to the stainless steel grades and their heat treatments. Chromium-containing steel grades, such as grades 410, 416 and 430, have strong corrosion scale.

All kinds of pickling processes will lead to metal removal to a certain extent. As a result, there is a considerable reduction in the brightness of the steel surfaces, and also in surface dimensions.

Carrying out heat treatment in a controlled environment and vacuum conditions, such as bright annealing, avoids the need for pickling operations and provides a better surface finish. However, pickling in sulphuric acid softens the corrosion scale, thereby allowing easy removal of rust particles through subsequent pickling in concentrated acids.

Table 2 below provides different kinds of pickling treatments for stainless steels.

Pickling Paste

Pickling paste is a paste-like mixture of strong acids that can be applied to overhanging surfaces, vertical surfaces or even small areas of stainless steels. It is one of the convenient methods of pickling used to remove heat tint after welding. Care should be taken while handling strong acids and residues, if any should be completely flushed into a suitable waste stream.

Some of the most commercial paste compositions are used in austenitic steel grades. While using pickling paste in lower alloyed steel grades, such as grade 3CR12, it is necessary to closely monitor the process to ensure the quick removal of paste, followed by thorough rinsing.

Typical pickling procedures covered under ASTM A380 are tabulated below:

Table 2. Pickling procedures. Refer ASTM A380

Notes:

1. Trial treatments should be performed to ensure that dulling is acceptable.
2. Pickling should preferably be carried out on fully annealed stainless steels due to risk of grain boundary attack. This problem is commonly observed in steels subjected to welding.
3. Thorough rinsing must follow all pickling treatments.
4. Observe all precautions for handling acids, such as sulphuric, nitric and hydrofluoric acids, as they are highly corrosive and harmful when exposed to skin.

Degreasing

Substances like lubricants, drawing compounds, cutting fluids, oil and grease should be removed from the stainless steel surface prior to heat or final passivating treatments. Components must be degreased using liquid or vapour degreasers before welding, to prevent carbon pick-up at elevated temperatures. Liquid cleaning can be carried out using hot alkaline detergents. The components need to be thoroughly mixed after rinsing.

Application of organic solvents to stainless steel surface can be carried out using vapour degreasing, swabbing or spraying, followed by rinsing in hot water. Like any other cleaning operations, the rate of cleaning can be improved with the use of stirring, jetting, brushing and so on.

Electropolishing

Electropolishing is a process of brightening the surface of stainless steels through electrochemical dissolution of high points. It is exactly the opposite of the electroplating process, but it is carried out using equipment similar to that of electroplating.

Electropolishing can produce a smooth finish. However, preliminary trials must be carried out to determine the polishing and surface condition parameters. The process can also produce frosted surfaces in some cases.

Grinding and Polishing

Although stainless steels can be easily buffed, polished and ground, certain properties of these steels need to be modified to achieve the best results. For instance, high strength is required for loading abrasive media, and low thermal conductivity is required to improve surface heat. This, in turn, causes surface smearing or heat tinting, and even re-hardening of martensitic grades, or sensitization of austenitic stainless steels in extreme conditions. Surface heat of stainless steels can be improved using methods such as: careful selection of lubricants to minimize loading of abrasives and use of lower feeds and speeds.

Polishing methods using coarse abrasives will generally have a greater impact on the corrosion resistance of stainless steels. Polishing with chromium oxide or alumina to attain smooth finishes, such as No.7 and No.8 buffed finishes, eliminates surface imperfections and fine pits, and enhances corrosion resistance. Buffing can also be performed with a "Scotch-brite" buffing wheel.

It is essential to remove iron contamination, in order to achieve good corrosion resistance of polished stainless steel surfaces. Abrasives and polishing products need to be free of iron, and the devices used for stainless steel processing should not be used for other metals. Under circumstances where these conditions cannot be satisfied, corrosion resistance of stainless steels can be improved using cleaning/passivation treatment.

Mechanical Cleaning

Mechanical cleaning can be used to overcome the problems related to chemical cleaning processes. In mechanical cleaning, care must be taken to prevent contamination of stainless steel surface by steel or iron particles.

Sand Blasting

Sand blasting is carried out with garnet sand or clean silica. Cut wire, grit and shot blasting must be performed with stainless steel media with corrosion resistance similar to that of the metal to be cleaned.

Wire Brushing

Wire brushing can be used to remove light tints formed from heat. Brushes are made from stainless steels and they should not be used on any materials other than stainless steels.

Barrel Finishing and Vibratory Finishing

Barrel and vibratory finishing methods are extensively used for mechanically polishing small sections of fasteners, such as pipefittings and bolts and screws, by using abrasive media. The mechanically cleaned products thus produced are not as corrosion-resistant as the ones produced by the pickling methods. Mechanical cleaning leaves behind certain scale residues, and it can be employed before acid pickling.

Blackening

Blackening is usually carried out using several processes, to produce a non-reflective black oxide surface. The two most commonly employed blackening methods involve immersing steels in a salt bath of molten sodium dichromate at 400°C, and in potassium dichromate/sulphuric acid solutions at 80 to 99°C.