diff --git a/lib/arel/visitors/bind_visitor.rb b/lib/arel/visitors/bind_visitor.rb
index c336e87395..8a5570cf5c 100644
--- a/lib/arel/visitors/bind_visitor.rb
+++ b/lib/arel/visitors/bind_visitor.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     module BindVisitor
diff --git a/lib/arel/visitors/dot.rb b/lib/arel/visitors/dot.rb
index 31e4a66a1f..85edc50eb7 100644
--- a/lib/arel/visitors/dot.rb
+++ b/lib/arel/visitors/dot.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class Dot < Arel::Visitors::Visitor
@@ -272,7 +273,7 @@ module Arel
             label = "<f0>#{node.name}"
 
             node.fields.each_with_index do |field, i|
-              label << "|<f#{i + 1}>#{quote field}"
+              label += "|<f#{i + 1}>#{quote field}"
             end
 
             "#{node.id} [label=\"#{label}\"];"
diff --git a/lib/arel/visitors/ibm_db.rb b/lib/arel/visitors/ibm_db.rb
index f1d126790d..e85a5a08a7 100644
--- a/lib/arel/visitors/ibm_db.rb
+++ b/lib/arel/visitors/ibm_db.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class IBM_DB < Arel::Visitors::ToSql
diff --git a/lib/arel/visitors/informix.rb b/lib/arel/visitors/informix.rb
index c33ef50554..b53ab18b82 100644
--- a/lib/arel/visitors/informix.rb
+++ b/lib/arel/visitors/informix.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class Informix < Arel::Visitors::ToSql
diff --git a/lib/arel/visitors/mssql.rb b/lib/arel/visitors/mssql.rb
index 92362a0c5f..8347d05d06 100644
--- a/lib/arel/visitors/mssql.rb
+++ b/lib/arel/visitors/mssql.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class MSSQL < Arel::Visitors::ToSql
diff --git a/lib/arel/visitors/mysql.rb b/lib/arel/visitors/mysql.rb
index ac3ad7b470..4c734f6292 100644
--- a/lib/arel/visitors/mysql.rb
+++ b/lib/arel/visitors/mysql.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class MySQL < Arel::Visitors::ToSql
diff --git a/lib/arel/visitors/oracle.rb b/lib/arel/visitors/oracle.rb
index 875b0e5b6a..99075b3e27 100644
--- a/lib/arel/visitors/oracle.rb
+++ b/lib/arel/visitors/oracle.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class Oracle < Arel::Visitors::ToSql
@@ -125,7 +126,7 @@ module Arel
             array[i] << ',' << part
           else
             # to ensure that array[i] will be String and not Arel::Nodes::SqlLiteral
-            array[i] = '' << part
+            array[i] = part.to_s
           end
           i += 1 if array[i].count('(') == array[i].count(')')
         end
diff --git a/lib/arel/visitors/oracle12.rb b/lib/arel/visitors/oracle12.rb
index 9b722e8c0c..ce90e994ae 100644
--- a/lib/arel/visitors/oracle12.rb
+++ b/lib/arel/visitors/oracle12.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class Oracle12 < Arel::Visitors::ToSql
diff --git a/lib/arel/visitors/postgresql.rb b/lib/arel/visitors/postgresql.rb
index ef0f0ea2ef..f0991a2f11 100644
--- a/lib/arel/visitors/postgresql.rb
+++ b/lib/arel/visitors/postgresql.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class PostgreSQL < Arel::Visitors::ToSql
diff --git a/lib/arel/visitors/sqlite.rb b/lib/arel/visitors/sqlite.rb
index 165e9cb6db..4ae093968b 100644
--- a/lib/arel/visitors/sqlite.rb
+++ b/lib/arel/visitors/sqlite.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class SQLite < Arel::Visitors::ToSql
diff --git a/lib/arel/visitors/to_sql.rb b/lib/arel/visitors/to_sql.rb
index 5429bf4ee8..3f61842c3f 100644
--- a/lib/arel/visitors/to_sql.rb
+++ b/lib/arel/visitors/to_sql.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'bigdecimal'
 require 'date'
 require 'arel/visitors/reduce'
diff --git a/lib/arel/visitors/where_sql.rb b/lib/arel/visitors/where_sql.rb
index 41972d5836..55e6ca9a21 100644
--- a/lib/arel/visitors/where_sql.rb
+++ b/lib/arel/visitors/where_sql.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 module Arel
   module Visitors
     class WhereSql < Arel::Visitors::ToSql