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The results are presented in the Appendix.  According to the survey, the most 

common kinds of surface contamination before the heat treating process are rust 

preventive oil and cleaner residue. After annealing, normalizing or carburizing, 

carbon deposits are the most important surface contminants. The oil and carbon 

deposit also existed after quenching. 

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2.2 Cleaning methods 

The cleaning process is often considered as a non-value-added process, compared to 

the processes such as plating, coating and heat treatment, etc. The cleaning process 

consumes time and money. Actually, the cleaning is very important for the quality of 

the end-products and it may influence subsequent manufacturing processes. [9] Hence, 

it is quite important to discuss the surface cleaning as being a fundamental step to 

reach best quality. During the heat treating process, cleaning may not recognized as an 

issue until after a thermochemical diffusion process (e.g. gas nitriding), even though 

the parts seemed visually clean. 

 

The processing issues or defects in the part that may arise due to not-cleaned surface 

are: 



 

Connecting layers of different thicknesses 



 

Different concentration profiles 



 

Partly or complete prevention of nitration in nitriding process 



 

Bad adhesion of connection layers [10] 

 

Some causes for defects could be type of material, geometry of the parts, positioning 

of the parts in the furnace, mechanical pretreatment and so on. If these parameters 

excluded, the defects are caused by non-visible diffusion restricting barrier layers on 

top of the working piece. [11] On the surface of a visibly clean part if there remain 

some residue, that may lead to a visible defect after heat treatment process. 

 

The rework of defective parts is difficult; it consumes time and money and, actually, 

may not always possible. The barrier layers have to be removed mechanically, for 

example by acid grinding or blasting. Afterwards the parts may need to be heat treated 

again. 

 

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Rust is attached to metal surface; the cleaning method to remove rust often contains 

metal loss, physically or chemically. For choosing the proper cleaning method for rust, 

there are several aspects that should be considered. [12] 

 

–

 

Thickness of rust or scale 

–

 

Composition of metal 

–

 

Allowable metal loss 

–

 

Surface finish tolerances 

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Shape and size of workpieces 

–

 

Production requirements 

–

 

Available equipment 

–

 

Cost 

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Freedom from hydrogen embrittlement 

There are various cleaning methods available to remove rust from the part surface 

[12-13]. 

Abrasive Blast Cleaning - Abrasive Blast Cleaning is widely used for removing all 

classes of scale and rust from ferrous mill products, forgings, castings, welding, and 

heat-treated parts. Depending on the requirement, abrasive blast cleaning can be the 

sole process, or combined with pickling, which is applied after to remove the 

remainder. [12-13,15,18] 

Tumbling – Tumbling is the least expensive method. However, the size and shape of 

parts are the limiting factors for this the process. Tumbling in dry abrasives is often 

used to clean small work-piece, and the parts with complex shape can not be descaled 

uniformly. Adding descaling compounds often decrease the required time by 75%. 

[12-13,16] 

Pickling  - Pickling in hot, strong solutions of sulphamic, phosphoric, sulfuric, or 

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hydrochloric acid is used for complete removal of scale from mill products and 

fabricated parts. Pickling is generally used as the second step after abrasive blast 

cleaning or salt bath descaling. At acid concentrations of about 3% and at 

temperatures of about 60

o

C or lower removed the steel. [12-14] 

Salt Bath Descaling - Salt bath descaling is an effective way to remove scale on 

alloys and tool steels.    Several types of salt baths either reduce or oxidize the scale. It 

operates at temperature range of 400 to 525

 o

C. [12-13,17] 

Alkaline Descaling - Alkaline descaling is more costly and slower in its action than 

acid pickling of ferrous alloys, but no material is lost using this method.    The action 

stops when the rust or scale is removed.    Immersion baths are usually operated from 

room temperature to 71

  o

C and can also be used between 93 to 99

o

C with a 

concentration of 20% alkali compound. [12-13,17] 

Acid Cleaning - Acid cleaning is effective to remove light rust, such as the rust forms 

on ferrous metal in storage under high humidity.    In acid cleaning, detergents, liquid 

glycol ether, and phosphoric acid are effective in removing the heavy oil compounds 

from the engine parts, even after it dried. By using a power spray, these acid solutions 

can clean the parts without manual scrubbing. Phosphoric acid cleaners may cause 

some discoloration, but it will not etch steel. Acid cleaners are usually used in a power 

spray. Some cleaners remove light blushing rust and form a thin film of protection 

temporarily. They are high in cost, but still often used in large ferrous parts, such as 

truck cabs. Phosphoric or chromic acid cleaners, with power spray or soak cleaning 

are used in removing most cutting fluids. These methods are expensive. But in some 

cases, they are still used because of their ability to remove light rust. [12-13]

 

Alkaline cleaning - Alkaline cleaning can remove the light oil compounds by 

immersion and spray cycles. The size of the parts is limited for alkaline immersion. 

The recommended minimum spray pressure is 0.10 MPa. The larger parts can be 

cleaned more effectively by spraying.  Alkaline cleaners [19] are efficient and 

economical for removing light oil and grease and can achieve to a no-water-break 

surface.  They remove oil and grease by saponification, emulsification or both. 

However, alkali will contaminate paint and phosphate coating systems, so thorough 

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